Dude there a bunch of threads here with that question. They use old refrigerators to run a circuit through or little radiators. Let me share this and do with it what you may - the secret is keeping the solution OUT of the reservoir. Simply running tubing in a huge loop as it goes from the res to the plants or vice versa is a start. As many feet as you dare and as your pump can handle. Keep in mind that as the line increases in length it increases the volume and, thus, lowers your pump pressure. Pipelines solve this by frequent compressor stations where the gas or liquid is gathered and re-compressed to a higher pressure and sent on down the line. In a grow room this could be as simple as another small reservoir with its own pump to boost that pressure in a long line. Or do the refrigerator bit.