Put some tap water into a container, let it sit overnight, and then test the pH. Tap water is often pressurized and thus the equilibrium level of gasses (most significantly CO2
) is different fresh out of the tap versus after it has some time to adjust to ambient atmospheric pressure. This could impact the pH. I suspect that once you let the tap water sit overnight, you will find the pH a little lower than 7.8. Might not be in the 6's, but probably in the lower end of the 7's rather than the upper end.
Any driftwood in the tanks? Tannins are acids, and so tanks with lots of driftwood often see a pH creep downwards.
The only other cause I can think of is the natural operation of the ammonia cycle. Remember that fish excrete ammonia, which is NH3
. When all is said and done and the biofilter has processed that, it is nitrate, NO3
. Which means each molecule of ammonia is eventually spitting out 3 hydrogen ions--acid.
If the cause is this latter process, then you have only a few options. You can reduce the bioload (fewer fish and/or increasing the size of the tanks). You can do PWC
's more frequently, or make them larger than you currently do (maybe 50% rather than 25%, though larger PWC
's also have the potential to be more stressful on the fish so that is a factor to consider). Or you can increase the buffering capacity of your tanks so that the acid generated by the nitrogen cycle is less able to impact the pH.
The latter option might be the easiest since you say you run a HOB
filter. I've never used Purigen in any tank so I'm not quite sure exactly what its purpose is, but my first reaction is to tell you to forget the Purigen and instead replace it with a small mesh bag of crushed coral. The fact that the filter water is flowing through the crushed coral will keep the water well-buffered. If you feel you absolutely need to keep the Purigen in the HOB
, then another option would be to take a mesh bag of crushed coral and just hang it in the tank, ideally near where the HOB
outflow flows back into the tank so that there is constant water movement around it. Back in the previous city I lived in my tap water was roughly neutral and had very little KH
(carbonate hardness, which accounts for buffering) and so my tanks were subject to some pretty wild pH fluctuations. Once I started hanging a bag of crushed coral near my HOB
outflow, the problem was eliminated. I just suggested putting it in the filter itself because, well, it's out-of-sight and not a distraction in the tank.
Please keep us posted on what you end up doing and the results.