I had some very surprising results today from my RO/DI unit. The unit was purchased a couple of years ago from PSI Water filters & was used for water top up for my marine coral aquarium. Mains water filtered through a double carbon cartridge using a TDS meter gives me 65 ppm. RO/Di reading is 0 ppm & waste water was 94 ppm. I played around with different mixes of RO + carbon filtered water. I filled a 2 x 18 x 18 tank with 80% carbon filtered water & 20% RO/DI to get 50 ppm which I believe is quite common for breeding. My pH is sitting at 6.8-6.9 but my meter needs calibrating. I have no kit to measure GH or KH but with a low TDS of 50 ppm, I assume the hardness would be quite low. I think we have pretty good water here for breeding & I may not need the RO at all. What TDS are you guys running for your breeding tanks. Grow out tanks are the same or a bit higher. Trickle system or sponges for about 24 breeding tanks? Looking at buying a Hi blow hp 100 or 200 if I go with sponges.