What makes my aquarium's ph levels go up?

High PH levels will stress out your fish and kill them. Adding filter water is the correct way to soften the water, but what type of filter water are you using? A simple solution to correct this problem is by adding "reverse osmosis" filter water also known as drinking water for people. Reverse osmosis water goes through various stages, first a Micron Prefilter Removes the larger partials, Micron Coconut Carbon Block with Housing removes medium partials and chlorine, Micron Prefilter for the cleanest water available into the membrane. This removes the maxim junk from the water and does not have carbon. Adding 1/4 tank RO water to your aquarium water will soften the water and buffer the PH levels with some PH down. PH level will remain constant and will not fluctuate. RO water can be purchased from your local water supply store, but remember to ask for RO water as they may carry other types of water. Just to let you know that PH down is a weak acid.