What are the tank dimensions (length x width x height)?
What is the GH (general hardness), KH (carbonate hardness) and pH of your water supply?
This information can usually be obtained from your water supply company's website (Water Analysis Report) or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
Depending on what the tank dimensions and GH of your water are, will determine what fish you should keep and how many will fit in the aquarium.
Angelfish, discus, most tetras, most barbs, Bettas, gouramis, rasbora, Corydoras and small species of suckermouth catfish all occur in soft water (GH below 150ppm) and a pH below 7.0.
Livebearers (guppies, platies, swordtails, mollies), rainbowfish and goldfish occur in medium hard water with a GH around 200-250ppm and a pH above 7.0.
If you have very hard water (GH above 300ppm) then look at African Rift Lake cichlids, or use distilled or reverse osmosis water to reduce the GH and keep fishes from softer water.
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Normal aquarium maintenance involves a partial (50-75% water change and gravel cleaning the substrate once a week. You should also do a big water change if you ever get an ammonia or nitrite reading in the water when there is fish in the tank.
Make sure any new water is free of chlorine/ chloramine before it's added to the aquarium.
An established biological filter should be cleaned at least once a month. However, if the filter is less than 6 weeks old, do not clean it because the beneficial bacteria are still settling into the media. To clean a filter that isn't an undergravel filter, wash the filter materials/ media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn. Undergravel filters get cleaned with a gravel cleaner when you do a water change. Cleaning the filter means less gunk and cleaner water with fewer pathogens so the fish will be healthier.
Wipe the inside of the aquarium glass down at least once a month and once a week or every couple of weeks is better. Cleaning the glass removes the biofilm, which is made up of various microscopic organisms, many of which can make the fish sick. You can use a sponge to wipe the inside of the glass down. Just get a cheap pack of sponges from a $2.00 shop and wash them with soapy water, then rinse well under tap water. Make sure the sponge doesn't have anti-mould agents or anything extra added to it. The cheap sponges don't normally have anything extra and have a yellow sponge on one side and a green nylon scourer on the other side. Use the yellow sponge side to clean the glass. When you clean the sponge before using it the first time, use a soap without perfume so it doesn't leave any residue. You wash the sponge before you first use it to remove any dyes in the sponge that haven't set properly. After you have washed it with soapy water the first time, you can just rinse it under tap water after that. you don't have to wash it with soap every time, just once to get the dye out.