The bigger the tank, the better it is for the fish. Tanks that are long and wide are better than tall narrow tanks. The smallest tank for neon tetras and virtually any tetra or other fish, is 2 foot long.
You can use peat moss to lower the pH. Put some in a mesh bag and put that in the filter. However, if the GH & KH of your water is high (above 150ppm), peat moss probably won't drop the pH.
If you have hard water, then mixing it with some reverse osmosis (R/O) water will reduce the GH, KH and pH, and you won't need peat moss.
Have a coverglass on top of the tank. Get glass that is 4mm thick because thinner glass (2mm and 3mm thick) chips and cracks easily.
Do not cover light units with material because they can heat up and catch fire.
You can put floating plants in the tank and they will provide shade for the fish. Water Sprite (Ceratopteris thalictroides/ cornuta) is one of the better floating plants and grows rapidly on the surface. It can also be planted in the gravel.
The GH and KH doesn't normally change much but it's still a good idea to check the GH and KH a couple of times a year to find out what it is. You don't need to buy test kits for this. Just take a sample of tap water to a pet shop twice a year and get them to test it for you. Some shops do it for free and others charge a small fee to cover the cost of the test. When they do test the water, write the results down in numbers and ask what the test is done in (ppm, dGH or something else).
If you are using mains water, contact the water supply company (via website or telephone) and ask them what the GH and KH are. Again write the numbers down at the time and find out what the tests are measured in (ppm, dGH).
Avoid buying sick fish. Neon tetra disease is sometimes seen in newly imported fish but is rare in the home aquarium. When you are at a pet shop, look in all the tanks and see if there are dead fish, any fish rubbing on objects, fish with swollen or cloudy eyes, fish with a cream, white or grey film or patches on their body. These are all signs of diseases and fish that are not healthy.
With neon tetras, look for the above list of symptoms, and look for any fish that has a faded blue or red line. If any fish in the tank has a faded blue or red line, do not get any fish from that tank. Go home and come back a few weeks later or visit another pet shop. Part of the blue or red line (usually near the middle of the body by the dorsal fin) will fade and go white if the fish have neon disease.
At home, do big regular water changes and gravel clean the substrate, clean the filter regularly, and feed the fish a varied diet to reduce the chances of them developing diseases. Quarantine all new fish for at least 2 (preferably 4) weeks before adding them to an established display tank.
It doesn't make any difference for neon tetras. However, the substrate should be as dark as possible and not white or yellow. The light coloured substrate will reflect light and stress the fish causing them to fade.
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