Fish are very adaptable to different pH levels. When you read those sites that give you ideal pH levels, they are referring to their natural environment. If you are buying your fish locally they are likely to have been bred and raised in tapwater similar to what you have.
What fish dont like is varying pH levels. The aim is to keep it consistent rather than what you might perceive as ideal. Adding Chemicals has the opposite effect, trying to move pH from where it naturally wants to sit it is constantly changing as it drifts towards its natural level and you then chemically alter it to a different parameter. Etc etc.
There are fish that are particularly needy and do require specific water parameters. Wild caught fish arent tolerant to different water parameters from their natural habitat. To get fish to breed you may want parameters closer to ideal. But in those circumstances its better to start with RO
water and remineralise it to the parameters you want rather than force tapwater to be somewhere it doesnt want to go.