I have a 3.5 gallon betta breeding tank. I used a ph decreaser on it and I dropped from 7.8 to 7.2 and its jumped back up again. It's a homemade tank and the water totally STINKS! I can't stabilize the ph and I know its not the decorations. Could it be water hardness? Pleeeeease help!

Use a water softener. Add some peat to the filter to lower the hardness and ph.