Tropical fish come from a range of pH levels in nature but most are happy in a pH of 7. 7 is classed as a neutral pH and is neither acid nor alkaline. Aim for a constant pH of 7 in your tropical aquarium and most popular tropical fish species should thrive. 

Exceptions are fish which naturally come from very acidic waters like Discus and Chocolate Gourami, or from alkaline waters like Lake Tanganyika cichlids and Sailfin Mollies. They may not thrive in a neutral pH so their water should have its pH value adjusted so that it's more to their liking, and they should only be mixed with other acid-loving or alkaline loving species.

What’s the pH of tapwater?

The pH of tapwater varies depending on where you live, and the bedrock that your water source is taken from. Water authorities also aim for a neutral pH in tapwater although it may vary from 6.5-8.5. Find out if you live in a soft water area (with low pH,) or a hard water area, (with high pH) as when you use tapwater to fill your fish tank and change the water, the pH will be affected. 

If your tapwater is very soft it will be perfect for most South American species like Tetras and Corydoras catfish. If you want to keep hard water fish but have soft tapwater you will need to use a pH buffer as well as a dechlorinator each time you do a water change.

If your tapwater is very hard it will be perfect for Lake Malawi and Lake Tanganyika cichlids, but if you want to keep soft water fish in hard tap water you’ll either need to use a pH buffer which lowers pH, or stop using tapwater altogether and instead use water which has been purified by Reverse Osmosis.

Abbreviated to "RO", Reverse Osmosis water has the added benefit of having chlorine removed,) so no liquid dechlorinator necessary and almost all the phosphate and nitrate is removed too, which can cause algae issues in tanks which don’t contain live plants.

But if you want an easy life test the pH of your tapwater using a pH test kit, and then match the fish to the pH.