Why my aquarium water PH getting lower and lower

[David Chan]

My tank is about 3 months old and I will change about 20% of the water every week. I noticed my guppies always cluster together except feeding time. I checked the PH value and realized it drop til 4.5 then I pour in solution to increase the PH value but didn't last long. My tank has only black sand, 5 lava rock and 2 drift wood. What went wrong?

[sthh]

1. Over a period of time, gunk (Waste and uneaten food) can be trapped in substrates, and even filter medium. These can lower the PH of your tank.
2. When there are more and more bacteria growing in your tank, the PH will also drop.

So what you can do to bring up the PH is
1. Vacuum the susbtrate, take out those rocks and drift woods and wash them. Change lots more water. You will be surprised how much gunk there are.
2. Rinse your filter medium.

To mitigate the risk of a PH collapse, you can buffer your tank with carbonate, I would suggest some coral chips. Or you can consider going for a barebottom tank for less maintenance.

[David Chan]

Thank you so much for your advice. I am planning to remove the black sand, laval rocks and the drift wood. The black sand really has plenty of gunk in them. You mention that when more and more bacteria growing, the PH of the water will drop. I thought the bacteria will remove ammonia and change to nitrate and nitrite. The nitrate and nitrite will be release as nitrogen gas then the PH value should not drop.

[sthh]

Nitrification by lots of bacteria does cause the PH to drop due to the creation of nitrite ions. But there are also other type of bacteria feeding on your gunk too.

[David Chan]

Understood. So having a barebottom tank will be a wiser choice to remove all the waste matter by filters. No more waste material residue to decay. Easier to maintain.

[sthh]

Yes, barebottom tanks are very much easier to maintain. However, there are lots of gunk and bacteria in your substrate. Move your fishes to a holding tank first. And removing all the substrate can also remove lots of beneficial bacteria too.