Water in an aquarium has minerals and salts dissolved in it. All these thing can be measured as TDS, Total Dissolved Solids.
When water evaporates, the minerals and salts stay behind. Now there is less water so the minerals and salts are more concentrated. TDS is higher.
You can top off with tap water. Tap water generally has minerals and salts. When you add this to a tank, to top off a tank where water has evaporated, this adds more minerals and salts. Then more water evaporates, TDS rises more.
Each time you do this there are more and more minerals and salts added to the tank. TDS keeps rising.
You can top off with water that has almost no minerals and salts. Reverse osmosis, rain water, and distilled water have almost no minerals or salts. When you top off the tank with this sort of water the TDS concentration drops down to almost as low as it was before any water had evaporated. The rise in TDS is so low you cannot read it with hobby level test kits. You could keep on topping off for a long time before you can see the rise in the test results.
This is very important to the livestock in the tank.
Fish regulate the levels of water, minerals and salts in their cells very carefully. When the TDS in the tank water is lower than the fish are accustomed to more water enters the fish cells, and the fish metabolism needs to work harder to get rid of it. When the TDS in the tank water is higher than they are used to their metabolism gets used to not having to work so hard. However, fish take in the minerals they need from the water. Certain fish evolved in water with very low mineral levels, so their body is very good at taking the minerals out of the water. When there are more minerals in the water (harder water) than they evolved in they still take out minerals from the water, and hold them in their body. They can hold more minerals than are good for them. Cardinal Tetras are one fish where this has been proven. They can die of excess calcium in hard water.
So, lets say you have been topping off with tap water, adding minerals each time. TDS is higher than the tap water. Then you do a water change with softer water (lower TDS). Gradually, as the TDS has been rising over several weeks or longer, the fish have been getting used to the harder water. Now, suddenly, the TDS is lower, and the fish metabolism has to ramp up to the level required to keep the water, minerals and salts in their cells in the right balance. The fish have a hard time doing this, and may not be able to remove all the excess water. This leads to dropsy.
This may be one reason why fish have a hard time when you buy new fish and add them to a tank with different water than they were used to. They can handle the higher TDS water (within reason), but the lower TDS water is harder to handle.
