What’s the right ph for your fish tank should be? is a question many people ask. Today almost all fish you buy in an fish store is farm raised, and they grew up in common tap water(chlorine, chloramine, and toxic metals removed of course). I once bought kuhli loaches and they died fast. I don’t know if it was because of the rock substrate instead of sand, or the water hardness. I spoke with a woman who sells kuhli loaches and said they die too in their store. So maybe they are used to more acidic water.

South American cichlids had not encountered any incident in my fish tanks, and they live in tap water with ph 7.4, 7kh carbonated hardness, and 11gh general hardness. Those fish tanks have lots of driftwood.

The only valid reasons I find to lower the ph in a tank is because you want to have extremely sensitive fish who only survive with lower ph in water, or because you want to breed fish who only does it when water is more acidic(like tetras, rasboras, barbs, South American cichlids, etc). And third, because you want live plants to behave superb. 

The only problem I find in maintaining a fish tank under these conditions is if the tank is too big. Because higher water volumes with lower acidity means more reverse osmosis gallons. If you have a high production RO filter installed in your house will be wonderful, but you’ll have to pay for that installation and modifications in your pipe system . This way you can exponentially filter RO water in higher numbers per day,  and fast(and still you’ll pay for minerals like seachem equilibrium  to increase water hardness every time you do water changes which will cost money every time). But for most of us, that is not an ideal solution. So we simple use a common reverse osmosis filter. 

In my case a 30, or 40 gallon tank will be enough. Then I use equilibrium from seachem to balance minerals to the new water added. That way I only change 10 gallons of water at most every 10 days. 

The advantage of having lower general hardness and carbonated hardness in an aquarium with plants is that plants absorb some nutrients in softer water that is imposible to absorb in harder water , there are some minerals and trace elements that cannot be absorbed in harder water, and any fertilizer you add to the aquarium will be not absorbed in totality  giving plants a hard time in certain conditions. Especially picky aquarium plants will not behave well, and even die. 

I just did a reading of my planted 30 gallon tank, and it gave me a ph of 7.4, carbonated hardness of 7 Kh, and general hardness of 11 Gh. It’s in a middle ground right now , but I have room to make the water softer, and I will. I’ll be adding reverse osmosis water to lower both water hardness, and decrease ph.

Harder water may lower ph if you add driftwood, liquid carbon, and moss(and only by a minimal fraction of a point in my experience) . But won’t decrease hardness in water, so any buffer will rebound. The only way to keep lower ph without buffering is to decrease carbonated hardness.