Hi and welcome to the forum
If you are getting lots of green algae on the glass or in the water, reduce the light and do some 75% water changes. Algae grows from too much light or too many nutrients in the water. Reducing the light should make a huge difference and adding some live plants will help. The plants will use the light and nutrients so the algae can't.
Some good plants to try include: Ambulia, Elodia/ Hydrilla, Hygrophila polysperma & rubra, Ludwigia, common Amazon sword plant, narrow Vallis, Water Sprite (Ceratopteris thalictroides/ cornuta).
Water Sprite is a floating plant that can also grow when planted in the gravel. The other plants all grow in the gravel.
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If you are losing fish, do not add any new fish until we work out what is going on.
If you got a new aquarium it will require a month or more for the filters to develop colonies of beneficial bacteria that keep the water clean and safe for the fish. While the filters are developing these bacteria, you get ammonia and nitrite building up in the water and these will kill fish very quickly. Washing the filter out with tap water can also cause problems to the filter bacteria.
What sort of filter do you have on the tank?
Is the filter run continuously (24/7)?
How often do you clean the filter and how do you clean it?
How often do you do water changes and gravel clean the substrate, and how much water do you change?
What are the tank dimensions (length x width x height)?
Have you checked the ammonia, nitrite & nitrate levels in the tank water?
Do you know what the general hardness (GH) and pH of your water is?
You can usually find the GH & pH by contacting your water supply company (phone or website). Alternatively you can take a glass full of tap water to your local pet shop and they can test it for you. Write the results down in numbers when they do the test. You can also buy your own test kits from a pet shop or online and test it yourself. If you do get test kits, try to buy liquid test kits rather than dry paper strip test kits. The liquid kits are more accurate. Check the expiry date on any test kits and keep them cool and dry.