I live very close to you Gvilleguy as you know. My water reads the same as yours minus the ammonia. The only way that I have found to keep the pH stable is to use crushed coral in the filter. Many people will say that it's not good to do so, but in our situation it can be an asset. While your bacterial colonies will adapt to the lower pH, they still will grow and function more slowly than if they were in a higher pH. A pH of 6.0 seems to be the magic number. Adding a bag of crushed coral to your filter will slowly buffer your water keeping it at around pH 6.0 raising slowly the longer you go between water changes. If you add crushed coral you will need to set a concrete water changing regimen so that you can change the water before the pH (hardness really) raises too far from your tap to prevent hardness shock. If there are no minerals in the water for a long period of time, your fish could suffer from bone and scale deterioration. Adding crushed coral adds calcium and magnesium, the very minerals that are the building blocks of those. I understand that fish get a lot of the nutrients they need from their food, but they also absorb some of them from the water that they are in. This is particularly important for fish that did not come from soft water ie: African cichlids and other hard water fish.
With all that said, you can also keep your hardness stable by performing many water changes. This method is much more labor intensive than adding crushed coral though.
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