I have had DIY CO2 running on my 400 litre tank for about 5 months now. I wasn't happy with the CO2 dispersal within the tank as I only had the gas bubbling out of an air stone. I bought one of these kits for the diffuser alone. The pH of the tank has dropped to, as far as I can tell, about 6. My test kit isn't that great so I can't tell the exact value.
Anyway, I only noticed the drop because I have just bought 4 inch long angelfish. They are just such great looking fish. Anyway during aclimatisation the floating bag decided it had had enough of floating and promptly sank. So much for a steady water adjustment!
Within an hour it was clear that the fish were struggling. This morning three of them were in bad shape with one having its slime coat peeling off. I have reduced the CO2 system to one 1.5 litre CO2 generator. The fish are looking a bit better. All the original fish are fine, rather active actually! I just think a 1.0 pH drop really shocked the poor new guys.
Anyway, the reason I posted here is to say that these diffusers are REALLY good. very efficient units. I know my water is soft but I didn't expect a 1pH drop in a 400 litre (88 UK gallon) tank with only one unit.
Be warned!
WK