Don´t worry about biological aspects - especially if you use copper blocks
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. Your only concern should be corrosion. Algae an bacteria will not grow in a bad habitat like a watercooling loop if your handle all parts of the loop at least with rudimental care to hygenic rules. Additives like PT-Nuke can bring up more problems than they supposable solve (btw - a cooper sulphate solution is most times cheaper if you buy it in a drugstore). Direct contact between pure destilled or demineralized Water and uncoatet copper, will lead to enough solute copper-ions, that they will do their job on killing all microbial life in the loop. Pretty like silver-ions, copper-ions are a good biocide, wich does not need any aid from additional coolant-ingredients. In comparison to additional silver kill-coils which will increase all corrosion proecesses, copper-ions are not avoidable with uncoated copper blocks, but they will not lead to more corrosion than normal.
With respect to corrosion szenarios, most glycol based coolant additives which contain some potent corrosion inhibitors are adequate. If your focus lies on an a clear coolant, Innoprotect IP should not be the worst choice. This is one of the most well proven glykol based coolant mixtures in the german watercooling scene als well als aquacomputer double protect ultra.
Glycol does not help preventing corrosion itself but it is neccessary to dissolve the corrosion inhibitors. On the other hand glycol brings in some slightly positive as well as a negative secondary effect. The positives are: increased viscosity wich supports the load capacity of the pump-bearing, lowered surface ernergy with reduces bubbleformation in the reservoir and, even if it is not really neccessary, glycol works also as a biocide. The negative effect is: glycol is not only a good solvent for most potent corrosion inhibitors, but also a good solvent for plasticizers in PVC-tubes. Glycol based coolant solutions therefore tend to increase the natural dissolution-rate of plasticizers in a relatively strong manner. Especially very soft material will react with a great loss of plasticizers in a short time, if glycol is supporting this process. Generally higher glycol concentrations lead to faster dissolution of plasticizers, which can end up in a blocked loop. The washed out plasticizers often compose to slimy deposits within the most narrow flow-channels, which are usually located in the cooling blocks. The second bad effect is found within the tube-material itself, as it gets more and more milky an unclear, when the plasticizers get lost.
This flaw of glycol based coolants respectively coolant additives could only be weakend by using relatively low concentrations, if there are no great corrosion-potentials in the loop to work againts. As copper is located clearly above the hydrogen electrode and also over all commonly used other metals within the electromotive series, the other metals in the loop are most vulnerable to erosive corrosion. As long as the potential gap between copper and other metals is narrow, or the other metals, like for example nickel, passivate very strong, even if the metal itself has not a good corrosion resistance, low corrosion inhibitor concentrations should not be kind of a problem. Only if there are materials like aluminium in contact to the coolant, high corrosion inhibitor concentartions are neccessary to prevent these parts from corroding down very fast. In such cases you shoud always keep an eye on the loop an replace the coolant relativley often, but the much better solution ist banning aluminium parts completely.
From this background you could come to the conclusion that it would be a good idea to dispense with glycol as an coolant ingredient, and use the corrosion inhibitors with no solvent instead. But as the past has shown that not one of these attempts was nearly as successful als any glykol-based solution. It is neccessary that the inhibitor molekules are dissolved in a homogenous manner within the complete coolant volume, and this can be reached by using glycol as solvent with comparatively small negative side effects.