Are all the cherry barbs red?
If yes they are all male and the one hiding in the plants is being bullied. A bigger tank (which you plan on doing) and a few females could change things, but at this stage, it appears to be taking cover in the plants to avoid conflict.
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300 mg/l (milligrams per liter) is the same as 300 ppm (parts per million). You have pretty hard water
Unless you are really attached to the cherry barbs, you might be better off keeping livebearers (mollies, swordtails, guppies and platies) or African Rift Lake cichlids. However, these will need a bigger. You can get a reverse osmosis (r/o) unit and connect it to a tap. The tap water will get forced through various filter media and that will remove most of the minerals from the water. The purified water is collected in a bucket and used at a 50/50 ratio with tap water to drop the GH to about 150ppm.
The drawback to r/o units is they tend to waste a lot of water. The good units have a 1:1 ratio of waste water to purified water. The waste water gets poured on the garden because it has all the minerals and stuff from the tap water. The lower grade r/o units can have a 2:1, 3:1 or 4:1 ratio and you get 2, 3 or 4 times the amount of waste water for 1 lot of purified water. If water is cheap then this isn't a big issue, but if you live in a dry climate or where water is expensive, then a r/o unit might not be the best option.
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SOLAR STILL
If you live in a warm climate, you can make a solar still and get pure water that way.
Get a large plastic storage container and put it outside in the sun.
Pour a bucket of water into the storage container.
Put a clean bucket in the middle of the storage container. Have a rock in the bucket to stop it floating around.
Put the lid on the storage container.
Put a rock or small weight on the lid in the middle, so the lid sags above the bucket.
As the sun heats up the container, water will evaporate and condense on the underside of the lid. The water will run towards the centre and drip into the bucket. When the bucket is full of water, you put it into a holding container and put the bucket back in the storage container with another bucket of tap water.
You get pure water with a pH of 7.0, 0 GH, 0KH and no wasted water, no power used and it's cheap to set up. You can then use this distilled water to mix with tap water to lower the GH and bring it into an range that is better for the cherry barbs.