acidic tank water

Question
Hi,
I have a 10 gallon fish tank that is sort of my personal little hobby. I call it my "free standing chemistry project". It's not a big deal and I don't have lots of time and money invested, but I do like to have a healthy tank of pretty fish to enjoy. I have three  tetras (glowlights, I think) and three platys. I was testing pH at around 7 at Christmas, but since then it has slowly dropped to 6.4. I use bottled water from the store because our tap water is acidic. I had an algae eater in there, but found him dead today. I've been replacing the water at about 2+ gallons every two weeks. I don't see this making any difference in the pH.  Questions: Should I be replacing water weekly? Do these types of fish do OK in the 6.4 pH? Could the food I fed the algae eater been part of the pH problem? Am I perhaps feeding the fish a little too much food, causing increased pH? I read about the soda/Epsom salt/marine salt recipe, but I don't have marine salt here, so guess I'll have to go to the store for that, or for some commercially prepared buffer. Suggestions?

Answer
Hi Jan,
You may be surprised to hear that oftentimes it is best to simply leave your ph where it is at now than try to adjust it. Trying to adjust it can cause much more harm to your fish than maintaining a stable ph level. This is because it is quite difficult to alter and maintain a specific ph level. You can have a roller coaster rides with the ph swings and some very stressed fish. One thing I'd do especially is ask your local pet store what ph level they keep their tanks at. Likely it will be the same as yours. And if so, you know that likely you will not have to alter your tank's ph level in order to keep any fish you bring home.

But one important key is this.. If your ph is dropping over a slow period of time it may be because the tank needs more partial water changes to maintain the buffering level in the water. Otherwise, if water sits too long... Acids produced by fish waste and food particle decay lowers your ph level. If you have any driftwood in the tank this too produces tannins and acids which lower your ph.

See what happens if you simply start changing two gallons twice a week with regular tap water. If the ph still is dropping lower and lower you can add some crushed coral to the substrate or your filter which will help produce buffers to keep your ph level a little more stable.

Fish can adapt to low ph but i wouldn't want it to get below 6.0 ... For the fish you have, they would prefer it higher.

Consider these things I've mentioned and hopefully we can solve this situation!
I hope this helps and best of luck!
Susan~