Hi and welcome to the forum
Water conditioner (dechlorinator) gets added to new tap water to neutralise chlorine or chloramine in the water. This is done before the new water is added to the aquarium. Contact your water supply company to find out if you have chlorine or chloramine in your drinking water, because they are treated slightly differently (you normally add more dechlorinator to treat chloramine).
If you don't already have a couple of fish buckets, go buy a couple of new buckets and use a permanent marker to write "FISH ONLY" on them. Use those buckets for the fish tank and nothing else.
You should do a 50-75% water change and gravel clean the substrate once a week. When you do a water change, fill up the bucket with tap water and add the required amount of dechlorinator to treat that bucket of water. Then aerate the mixture for at least 5 (preferably 30) minutes before adding the water to the tank. The aeration helps the dechlorinator come into contact with all the chlorine/ chloramine molecules in the water and neutralise them before the water is added to the tank. Aeration also helps get dissolved gasses in the tap water, back to normal levels.
You should also monitor the ammonia, nitrite and nitrate levels in the aquarium water and do a 75% water change any day you have an ammonia or nitrite reading above 0ppm, or a nitrate reading above 20ppm. You need ammonia and nitrite to stay on 0ppm, and you want nitrates to stay as close to 0ppm as possible, and under 20ppm at all times.
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Aquarium filters should be cleaned at least once a month. However, if the filter is less than 6 weeks old, do not clean it. When you do clean a power 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.
If your filter has pads/ cartridges in it, and the instructions tell you to replace them every month, disregard the instructions. That is a gimmick by filter companies to make you keep buying their products.
If you do have filter pads/ cartridges in the filter, you can squeeze them out in a bucket of aquarium water and re-use the pads. Do this until the pads start to fall apart, then replace them.
You can put sponge in the filter instead of pads/ cartridges. I used AquaClear sponges but there are numerous other brands and they all do the same thing. Just find a sponge that is slightly bigger than the filter pad and cut the sponge down to size with a pair of scissors. You can normally add sponge/s while there are filter pads in the filter and they get squeezed out in a bucket of tank water too. Sponges will last for years and are a better option than filter pads and cartridges.
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I prefer to keep guppies, mollies, platies and swordtails in single sex tanks (all males or all females) so the males don't continually harass the females.
If you have an aggressive fish, put it in another tank or get rid of it.
