I am in an area in KY where the water is very hard, straight from the tap. It's about 280ppm. I have some limestone rocks in my aquarium that bump it up even more. And on top of that, I have been topping my tank off every few days due to evaporation. I have no cover on the tank, which might be an issue. My water right now is around 500 ppm, using my tds meter. How high can it go without affecting my africans?