A pH of 6 is far too low. Your major problem is that biological filtration proceeds best at pH 7.5 to 8, and below pH 6, stops altogether. So whatever else happens, you're going to have trouble maintaining good water quality. Now, when people keep fish at pH 5 or pH 6, as they sometimes need to (e.g., for chocolate gouramis) then expert fishkeepers at least forget about biological filtration and simply use zeolite. A bubble-up box filter stuffed with zeolite should last a week, and you can change the zeolite as often as required. Since it can be recharged -- up to a point, at least -- it is a cost effective way to maintain lightly stocked soft water tanks.
If this doesn't appeal, then you need to raise the carbonate hardness. Do that, and pH will follow. There's a recipe for making Rift Valley cichlid salt mix that's been about for years, and I have a copy
on my web site. You won't need the full dose, and will probably find a half dose, or even a quarter dose, adequate. In other words, where it says one teaspoon or tablespoon, add one-half or one-quarter that amount.
What you're after is a pH around 7 (6.5-7.5 is fine) and a carbonate hardness of about 3 degrees KH or more. The general hardness should be around 10 degrees dH. for most community fish, but a little below or above will not make any difference.
As you should know, guppies need hard, basic water, and won't be happy if the pH is below 7 and the hardness is below 10 degrees dH. Tetras, gouramis and most catfish are less fussy, but often prefer water that isn't too hard, around pH 8, 20 degrees dH being about the maximum for most species to do well.
Make water chemistry changes slowly, ideally changing 20% of the water per 24 hours, taking time to thoroughly dissolve into each bucket of water all the chemicals needed before use.
Cheers, Neale