Hi and welcome to the forum
I know nothing about fish. This is a first pet for my son who lost interest after the first day and now I am stuck with it!
LOL, I've heard that story plenty of times before. It's not always fish, it can be birds, cats, dogs, guinea pigs, etc.
Milky cloudy water is a bacterial bloom usually caused by fish food breaking down in the water and a filter that hasn't cycled.
Green cloudy water is an algae bloom caused by too much light, too many nutrients, or no live plants to use the light and nutrients.
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You can do a fish in cycle. This is where the fish stay in the tank while the filters develop colonies of beneficial bacteria.
Just reduce the feeding to 2-3 times a week and do a 75% water change and gravel clean the substrate (sand or gravel on the bottom of the tank) 4-8 hours after feeding. When the filter has finished cycling (developing the colonies of good bacteria), you can feed the fish more often and do a water change and gravel clean once a week. Don't worry, the fish won't starve during that time.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank. Get a couple of new buckets and use a permanent marker to write 'FISH ONLY" on them. Use those buckets for the fish and nothing else.
Before you do a water change, add enough dechlorinator to the bucket to treat that bucket of water. then fill it with tap water. Stir it up or aerate it for at least 5 (preferably 30) minutes and then use that water to fill the tank up after you have removed some of the dirty water.
You can buy a gravel cleaner from any pet shop and use it to syphon some water out of the tank, and to clean the gunk out of the gravel. The fish and gravel stay in the tank and you simply use the gravel cleaner to suck the gunk out of the tank and into a bucket. Take the bucket of dirty water outside and pour it on the lawn or garden.
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Try not to clean the filter for the first 6-8 weeks. This gives the good bacteria a chance to stick to the filter media/ materials. After that you can clean it once a month.
When you do clean the filter, wash/ squeeze the filter media out in a bucket of tank water. Then re-use the media. Tip the bucket of dirty water on the lawn or garden.
Filter materials should only be replaced when they start to fall apart. If that happens, replace them with a sponge for another brand of filter. You can sue a pair of scissors to cut the sponge so it fits in your filter. Sponges get cleaned the same way and last for years.
If you have carbon (black granulated substance) in the filter, you can remove this and don't bother replacing it. The same with Ammogon/ Zeolite (white granules). This stuff removes ammonia from the water and stops the filter bacteria from growing.
The filter case can be washed out under tap water, as can the impellor and impellor assembly. The impellor is the black magnet with 3 or 4 plastic blades on one end and sits inside the motor of the power filter. Check the instruction book for directions about cleaning the impellor.
If you have any concerns, just post questions and pictures here and we can help you work it out.