Ph Gone Low

The main cause will be your water supply. It sounds as though you have very low carbonate hardness (KH). Carbonate acts as a buffer which keeps pH stable. If there is not much of it, the small amount there is gets used up fast. Bogwood also helps lower pH, especially if the carbonate level is low. Is the wood releasing a lot of tannins (that is, is your water going very brown)?

The other possibility is that the fish you have just added have pushed your tank to the limit. The end product of the nitrogen cycle is nitrate, which is acidic. If you are now heavily stocked, your fish would be producing a lot of ammonia which would be turned into a lot of nitrate. This, coupled with probable low KH and the bogwood, might be enough to cause the pH drop.

And it could well be lower than 6 if that is the lowest colour on your tester.


You can confirm the KH of your tapwater (not tankwater) either by purchasing a tester or by getting it checked at a shop. Get them to tell you the actual number, and also what unit they are using as it could be degrees of hardness or ppm - the two give different numbers.
If it turns out the KH is low, you need to do something to increase it. You may be unwilling to remove the bogwood, though that would help, but the other thing is to add calcium carbonate, which will slowly dissolve, raising the KH. It will also raise the GH and increase the pH slightly.
You could: put some crushed coral in your filter, use coral sand in your substrate if you have sand, put a piece of limestone rock in the tank, put shells in the tank. I would also do more regular, smaller water changes than usual as you don't want to add a lot of new water at one go as you would remove a big chunk of the dissolved calcium carbonate with a big water change.
One other option would be to add remineralisation salts of the type you add to RO water. You would add it slowly so as not to shock the fish by changing the hardness too quickly - these salts also add to the GH and changing that too fast is not good for fish - then when the tank water was at the level you want, add it just to the new water at every water change. You would need to buy a tester if you choose this route as you need to know how much of the salts to add until you find the right amount.

There are some plecs that need wood in their diets. I'm not well up on what plec needs what, but yours might well be one that needs wood.

The coral will dissolve slowly adding both calcium (and therefore increasing GH a bit) and carbonate (and increasing KH). GH is more important for fish than pH and you want to minimise the amount that GH changes by. During a water change you will be removing some water with a fair amount of calcium and replacing it with water that has less calcium. Smaller, more frequent water changes mean you will be changing the calcium level less in one go that if you did big, less frequent ones.