The Tool Desk
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This guide is for refillable custom loops. A typical sealed all-in-one (AIO) cooler is not meant to be opened or refilled; doing so can damage it or void its warranty.
What PC coolant does—and what it cannot promise
Coolant is the working fluid that carries heat from the CPU and GPU blocks to the radiators, where fans move that heat into the air. A purpose-made PC coolant also contains additives intended to inhibit corrosion and biological growth and to limit deposits, residue, foaming, or staining. Its formulation must suit the loop’s metals, plastics, tubing, seals, and coatings.
Do not choose a fluid solely because its packaging promises better cooling. Radiator capacity, airflow, pump flow, block mounting, fan speed, room temperature, and component power draw usually have more influence on temperatures than modest differences between reputable coolants. There is no independent, controlled comparison establishing one of the products below as the universal thermal winner. In practice, a compatible coolant that stays clean and does not obstruct a block’s fine channels is more useful than a flashy fluid with avoidable maintenance problems.
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Quick recommendations
| Coolant | Best fit | Form and appearance | Key qualification |
|---|---|---|---|
| Aqua Computer Double Protect Ultra | Clear premix for a compatible custom loop | Ready to use; clear | Aqua Computer recommends annual replacement. Its German-store price in August 2026 was €8.98 for 1 liter or €32.90 for 5 liters, including German VAT and excluding shipping. |
| Corsair XL5 or XL8 | Corsair Hydro X systems | Premix; clear and colored variants | Corsair specifies copper/brass compatibility and a maximum coolant operating temperature of 60°C; check the Hydro X compatibility list. |
| Koolance LIQ-702 | Builders who want a clear premix with published specifications | Ready to use; clear | Koolance lists conductivity of approximately 2,500 µS/cm, freezing point of −15°C, and viscosity of 2.31 cP at 20°C. Verify loop compatibility and warranty terms. |
| Koolance LIQ-705 | Users prioritizing low stated conductivity | Ready to use; clear | Koolance lists approximately 3 µS/cm conductivity and a −35°C freezing point. Low conductivity does not make spills on powered hardware safe. |
| EK-CryoFuel Clear | EK-oriented builds or builders who prefer a concentrate option | Premix or concentrate; clear | The concentrate listing gives a pH range of 6–8 and corrosion-inhibitor information. Follow the exact current product instructions for dilution; do not infer a ratio. |
Which coolant should you buy?
For a first custom loop: clear premix
A ready-to-use clear coolant avoids a dilution calculation and makes it easier to spot cloudiness, particles, or unexpected discoloration. Aqua Computer Double Protect Ultra is a sensible starting point when every component’s documentation permits it. The manufacturer describes it for PC water-cooling systems, including systems with copper and aluminum, but that product-specific claim does not override restrictions from an individual block or radiator. Aqua Computer recommends changing it annually. Its German store listed €8.98 for 1 liter and €32.90 for a 5-liter canister in August 2026; those prices included 19% German VAT and excluded shipping, so they are not U.S. price quotes. See Double Protect Ultra’s product information and the 5-liter canister listing.
For Corsair Hydro X: XL5 or XL8
Corsair identifies XL5 and XL8 as preferred coolants for Hydro X. Both are premixed, with clear and colored variants in the range. Corsair says to use them only with copper/brass products and sets a maximum coolant operating temperature of 60°C. That limit applies to the fluid temperature, not the CPU or GPU core-temperature reading. Review the Hydro X coolant compatibility table and the product pages for XL8 and XL5. Corsair lists some third-party products as compatible and others as incompatible; a product’s popularity elsewhere does not establish Hydro X compatibility.
For published fluid specifications: Koolance LIQ-702
LIQ-702 is a clear, ready-to-use coolant with corrosion and biological inhibitors. Koolance lists approximately 2,500 µS/cm electrical conductivity, a −15°C freezing point, and viscosity of 2.31 cP at 20°C. It is offered in several sizes, including 700 ml, 5 liters, and 10 liters. Those specifications describe the product under the manufacturer’s stated conditions; conductivity can change as fluid absorbs contaminants in a real loop. Koolance’s warranty language excludes certain issues involving third-party coolants, additives, and corrosion-related damage, so check its terms before filling. Read LIQ-702 specifications and Koolance’s product and warranty information.
For low stated electrical conductivity: Koolance LIQ-705
Koolance describes LIQ-705 as electrically insulative relative to ordinary water-based coolants. Its 700 ml product listing gives approximately 3 µS/cm conductivity, a −35°C freezing point, and viscosity of 4.3 cP at 20°C. The official U.S. page listed $6.49 for 700 ml when checked. That low-conductivity figure is not a guarantee against electrical damage: a spill can pick up contaminants, and no coolant makes it safe to leak onto powered components. Choose LIQ-705 only after confirming it is suitable for the entire loop. See LIQ-705 specifications.
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- Ok for all plastics, rubbers, and miscellaneous gasket materials.
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For builders who want concentrate: EK-CryoFuel Clear
EK sells CryoFuel Clear as premix and concentrate. The concentrate is useful when you already have suitable distilled or deionized water and can measure accurately. EK lists a pH range of 6–8 and corrosion-inhibiting ingredients for the concentrate. Use the dilution ratio on the current product label or instructions; the available product information here does not establish a ratio to apply universally. EK warns that CryoFuel is not compatible with Mayhems Blitz cleaning solution. See the concentrate listing and premix listing.
Check compatibility before choosing a fluid
Coolant contacts more than the visible tubing. Make a list of every wetted part, then check both the coolant maker’s guidance and the documentation for each component. A restriction from any part of the loop is reason not to use that coolant.
- Metals: copper cold plates, nickel-plated copper blocks, brass fittings, copper or brass radiators, and any aluminum or stainless-steel parts.
- Polymers and seals: EPDM, PVC, silicone, other tubing materials, O-rings, acrylic, and acetal reservoir or block tops.
- Operating limits: the coolant’s temperature rating, plus any limits on flow, cleaning products, or additives stated by component makers.
- Warranty terms: whether the manufacturer requires or prefers a particular fluid, or excludes damage associated with third-party coolant or corrosion.
Mixed metals and aluminum
Putting dissimilar metals in one loop can create galvanic-corrosion risks. Some coolant formulations are intended for broader material combinations, but no general claim makes arbitrary combinations safe. Follow the strictest guidance from the coolant, block, radiator, pump, and fitting manufacturers. In particular, do not use a copper/brass-only coolant in a loop containing aluminum unless its manufacturer explicitly permits that configuration.
Nickel plating and cleaning chemicals
Nickel-plated blocks need the same product-specific compatibility check as other wetted components. Avoid aggressive cleaners or additives unless the block and coolant makers approve them. EK’s warning against combining CryoFuel with Mayhems Blitz is a reminder that a cleaning chemical and a coolant can be incompatible even when each is sold for water-cooling use.
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- Mayhems thermal performance focused premixed PC coolant for water cooling computer systems and loops.
- Mayhems longest operational lifetime coolant with great levels of thermal performance
- Prolongs the life and thermal cooling efficiency of your liquid cooling computer components
- Specially formulated to protect your liquid cooling computer components from corrosion and scale build up.
- Colour: Ozone Clear Blue (clear but turns ice white with a subtle ice-blue aura/haze under UV lighting)
Warranty and ecosystem compatibility
Use the exact compatibility list for the system you own, not a broad community recommendation. Corsair explicitly marks some fluids incompatible with Hydro X, including EK CryoFuel Solid, EK CryoFuel Mystic Fog, Mayhems SFX Aurora, PrimoChill VUE, and Thermaltake C1000 Opaque. A product listed as compatible by one ecosystem is not automatically approved by another manufacturer. Koolance also cautions that its warranty does not cover certain problems involving third-party coolant, additives, or corrosion.
Clear, colored, and opaque coolant
Clear fluid: easiest to inspect
Clear coolant is the most conservative choice for a first build, a workstation, a loop with fine microchannels, or a system that will be serviced infrequently. It makes it easier to notice suspended particles, cloudiness, or color changes that could signal contamination or corrosion.
Transparent colored fluid: an aesthetic trade-off
Transparent dyes can suit a carefully planned color scheme, but the exact dye formulation still needs compatibility approval. Color changes and staining may be harder to interpret, and residue can complicate cleaning. Choose it because the appearance matters enough to accept closer monitoring—not because color improves heat transfer.
Opaque, pastel, glitter, and special-effect fluids
These are aesthetic products, not a universal performance upgrade. They may call for more cleaning and inspection, and particles or residue can be unwelcome in fine channels. They are a poor match for a first loop, a machine that must run continuously, or a builder unwilling to disassemble and clean blocks. Corsair’s Hydro X table marks several opaque or effect fluids incompatible with its products, so check that table before considering any such fluid for a Hydro X loop.
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- Mayhems thermal performance focused premixed PC coolant for water cooling computer systems and loops.
- Mayhems longest operational lifetime coolant with great levels of thermal performance
- Prolongs the life and thermal cooling efficiency of your liquid cooling computer components
- Specially formulated to protect your liquid cooling computer components from corrosion and scale build up.
- Colour: Infinite Void Black
Premix or concentrate?
| Form | Advantages | Trade-offs | Best suited to |
|---|---|---|---|
| Premix | No dilution step; consistent, convenient fill; lower risk of using the wrong water or ratio. | Can cost more per finished liter and involves shipping ready-mixed fluid. | First-time builders and anyone prioritizing simplicity. |
| Concentrate | Can reduce cost per finished liter and is easier to store or transport before mixing. | Requires suitable distilled or deionized water, accurate measurement, and the exact current dilution instructions. | Experienced builders comfortable with careful preparation. |
Do not dilute concentrate with tap water: minerals and contaminants can enter the loop. Do not combine brands, additives, or leftover fluids just to use them up; mixed chemistry creates uncertainty. EK’s CryoFuel Clear concentrate and premix are distinct products, so follow the instructions for the one you purchased.
Is distilled water enough?
Not as a complete long-term coolant. Water transfers heat well, but plain distilled water does not provide the corrosion and biological protection expected from a purpose-made PC coolant. EK explicitly warns against using pure distilled water as the cooling liquid in its installation material. If you use distilled or deionized water as part of a concentrate or a manufacturer-approved system, use the specified corrosion inhibitor and biocide approach. Avoid automotive, aquarium, household, or other chemical additives unless every relevant component and coolant manufacturer explicitly supports them. EK’s installation material contains its warning on pure distilled water.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Conductivity and leak safety
Lower electrical conductivity may reduce the immediate risk if fluid reaches electronics, but it does not make a leak harmless. Water-based coolant can become more conductive after picking up dust or other contaminants, and a powered system can still be damaged by a spill. Koolance’s listed values—approximately 3 µS/cm for LIQ-705 and 2,500 µS/cm for LIQ-702—are product specifications, not predictions of conductivity after aging or contamination in your loop. See the LIQ-705 sheet and LIQ-702 sheet.
If a leak or spill occurs, shut the system down, disconnect it from mains power, and do not restart it until affected parts are fully dry and inspected. Leak-test a new loop with sensitive components disconnected from power. A leak-testing pump or other method should follow the pump and component makers’ instructions; there is no single test duration established for every setup.
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- Mayhems thermal performance focused premixed PC coolant for water cooling computer systems and loops.
- Mayhems longest operational lifetime coolant with great levels of thermal performance
- Prolongs the life and thermal cooling efficiency of your liquid cooling computer components
- Specially formulated to protect your liquid cooling computer components from corrosion and scale build up.
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How much coolant to buy
There is no reliable universal volume based on whether a loop cools a CPU, a GPU, or both. Radiator size and thickness, reservoir capacity, tubing length, block count, and the fill-and-drain layout determine the amount. Estimate from the actual loop, then allow extra for bleeding, accidental spills, and a future partial drain. Keep a sealed reserve if the coolant may be difficult to source later.
Filling and leak-testing a new loop
- Check compatibility: confirm the chosen fluid against every component’s materials and instructions before filling.
- Prepare the loop: clean and flush components according to their manufacturers’ procedures. A new radiator can still contain manufacturing residue; do not assume it is clean out of the box.
- Fill the reservoir first: keep the pump supplied with enough fluid before starting it. Never run a liquid-cooling pump dry.
- Move trapped air: tilt or gently rock the case as appropriate so bubbles can travel to the reservoir.
- Inspect connections: check fittings, terminals, drain ports, and tubing joints while filling and circulating fluid.
- Test before powering components: keep the motherboard, graphics card, storage, and other sensitive hardware disconnected from power during the leak test, and follow the pump and component makers’ directions.
- Stop at an abnormal change: if fluid turns cloudy immediately, develops unexpected sediment, or changes color, shut down and investigate instead of continuing operation.
When to replace coolant and what to watch for
Use the coolant maker’s maintenance guidance as the baseline, then respond to the condition of the loop. Aqua Computer recommends an annual change for Double Protect Ultra; that is a product-specific recommendation, not proof that every coolant in every system has an identical service interval. Replace fluid sooner if it becomes cloudy, develops particles, changes unexpectedly in color, smells unusual, or shows evidence of biological growth or corrosion. Drain and inspect the loop when a major hardware change requires opening it.
If coolant turns cloudy or changes color
Possible causes include radiator residue, an incompatible coolant or additive, breakdown of opaque particles, tubing plasticizer, corrosion, biological growth, excessive heat, or operation outside the fluid’s stated limits. A color change alone does not identify which cause is responsible.
- Shut down the system and inspect the fluid and loop.
- Photograph sediment, staining, or visible plating damage before cleaning.
- Drain the loop and inspect the reservoir, pump, block jet plate, and microchannels.
- Flush only with a procedure approved for the components and coolant; replace damaged tubing, seals, or heavily contaminated parts where needed.
- Refill with one compatible coolant rather than mixing the old fluid with a different brand or additive.
If the pump becomes noisy
New gurgling or rattling can indicate trapped air or inadequate fluid at the pump inlet. Stop the pump if it is not receiving sufficient liquid, check reservoir level and fill procedure, and follow the pump maker’s guidance. Do not run the pump dry to try to clear air.
If you suspect corrosion or growth
Stop using the loop and inspect it rather than topping up and hoping the issue clears. Deposits, plating changes, or persistent particles may require block disassembly and component-specific cleaning or replacement. Use only cleaning and flushing products approved for the affected materials.
Quick Recap
Buyer guide by build
- First custom loop: choose a clear premix supported by every component; prioritize easy inspection and avoid opaque effects.
- Corsair Hydro X: use Corsair’s preferred XL5 or XL8 where appropriate, observe the copper/brass-only guidance and 60°C coolant limit, and verify the exact fluid against Corsair’s table.
- Clear loop with published specifications: consider LIQ-702 if its conductivity, temperature properties, compatibility, and warranty terms suit your build.
- Low-conductivity priority: consider LIQ-705 only after component compatibility checks; it lowers stated conductivity but does not remove leak risk.
- Experienced, budget-conscious builder: a concentrate such as EK-CryoFuel Clear can make sense if you have suitable water and will follow its current dilution instructions precisely.
- Aesthetic enthusiast: use colored or effect fluid only when the exact formulation is approved for the loop and you accept the added monitoring and cleaning burden.
Product prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon at the time of purchase will apply.
