What exactly is the salt? I have no idea what is sold in Thailand, but in the UK where I live they sell aquarium salt which is just plain salt like you use in cooking, marine salt like you would add to a marine fish tank, and remineralisation salts that you add to RO water. Does your mineral salt sound like any of those?
The reason they sell 'aquarium salt' (plain salt) is that before we knew about the nitrogen cycle it was found that salt helped fish stay healthier. We now know that it reduced nitrite poisoning, and since we now keep nitrite at zero in our tanks we don't need 'aquarium salt'. Some fish are harmed by it, just less harmed than being poisoned by nitrite. But some fishkeepers still go by the old ways and add salt even though it is doing nothing to help the fish in a properly maintained tank.
Marine salt is obviously for marine and brackish tanks, though tanks with only mollies in them can also use it.
Reverse osmosis water (RO) has had all the minerals removed. But fish can't live in this kind of water; even soft water fish need a few minerals. So remineralisation salts put minerals back into RO water, and you control how hard you make it by how much you add.
If your mineral salt is the cooking salt type (sodium chloride) you don't need it. If it's the sea salt type, you don't need it. If it's the remineralisation type, you are making your water even harder by using it, so stop.
When rain falls it is almost pure water, it has only pollution from the air in it. It falls on the land and flows into rivers and lakes. The rock that the water flows over slowly dissolves, and some rocks dissolve better than others. The rocks that dissolve easiest usually have calcium in them, so water from those rocks has a lot of calcium in it. When we talk about hardness and GH, we are talking about calcium. The more that has dissolved, the harder the water and the higher the GH. Where the rocks hardly dissolve at all, like the rocks the Amazon flows over, very little dissolves so there is hardly any calcium in the water - so we get soft water and low GH.
The rocks that have most calcium are limestone and chalk. These are made of calcium carbonate. So not only does calcium get into the water but also carbonate, and when we measure carbonate it is called KH. Carbonate reacts with acids and stops the water becoming more acidic, that is it stops the pH dropping. Usually, but not always, when the water is hard (high GH) there is also a lot of carbonate (high KH) so it is very difficult to lower the pH because all that carbonate stops the pH dropping.
OK, it's really a bit more complicated than that! There are other minerals besides calcium and carbonate but those are the ones we measure with GH and KH, and they are the ones that make the water what we call hard.
The minerals that make the water hard usually also make the water alkaline, that is they make the pH high. So it is common to find that places that have hard water also have a high pH.
When water evaporates from the tank it leaves the minerals behind, so they build up and make the tank water GH even higher. If the tank is topped up with the usual water that will add more minerals, then more evaporates so it's topped up again adding more minerals and so on. Even doing water changes and taking water out doesn't help this problem as a lot of the minerals are left behind. When you add your mineral salts after a water change you are just helping the minerals build up faster.
Topping up with water that doesn't have any minerals in it would solve this problem of the minerals building up. Can you get distilled water or RO water where you live as these would be the best for topping up? Not for doing water changes with, just for topping up. If adding the bottled water makes the tank pH that low, it is not good for the fish to change it so much. Or did you mean the bottled water had a pH of 5 or 6? Besides, what minerals are in the bottled water? You could be changing the mineral content of the tank water so it is different from your tapwater so that when you do your weekly water change, the new water won't have the same minerlas as the tank water, which could affect the fish.
What I would do in your circumstances would be first of all stop adding the salts. I would choose fish that can live happily in water with a high GH and pH - research before you buy. I would top up with distilled or RO water, adding a bit of water quite often rather than wait till a lot had evaporated then add a lot of water. When you do your water changes, I would use whatever water it was when you started the tank as that's what the fish in there will be used to. Topping up often with small amounts of pure water (distilled, RO) means that the small amounts won't change the water very much after each addition and when you come to do the weekly water change the mineral content of the tank will still be the same as your tapwater.
