Hi and welcome to the forum
What are the tank dimensions (length x width x height)?
Don't add any more fish, shrimp or snails to the aquarium until it has cycled.
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You are probably doing a fish in cycle, which simply means there are fish in the aquarium while beneficial bacteria develop in the filter. It usually takes around 4-6 weeks but can take a bit longer.
During this time you should reduce feeding to 2-3 times a week. Don't worry, the fish won't starve. Unlike terrestrial animals they use most of the food they eat to keep warm, most fish take their body temperature from the surrounding water, so any food they eat goes into growth and movement.
You should do a 75% water change and gravel clean the substrate 4-8 hours after feeding. And do a 75% water change any day you have an ammonia or nitrite reading above 0ppm, or a nitrate reading above 20ppm.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Continue doing this until the ammonia and nitrite have both gone up and come back down to 0ppm. Then the filter will have cycled. At which time you can feed the fish once a day and do a 75% water change and gravel clean the substrate once a week.
Don't bother testing for nitrates until the ammonia and nitrite have both gone up and come back down. Nitrate test kits read nitrite as nitrate and give you a false reading. Wait until the filter has cycled and then start testing for nitrates.
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If the aquarium water is a milky cloudy colour, that is normally caused by a new tank and uneaten fish food on the substrate. Do a big (75%) water change and gravel clean every day until it clears.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
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Goldfish need plant matter in their diet and at least half of their diet should be plant based. You can feed them vege flakes/ pellets, as well as fruit n veg like cucumber, zucchini, pumpkin and marine algae available from Asian supermarkets or in the Asian section of a normal supermarket.
You can also grow Duckweed on the surface of the aquarium or outside in ponds. It is a small floating plant and most goldfish love it. Growing aquatic plants in the aquarium will give the goldfish something to graze on. Ambulia, Hygrophila polysperma and narrow Vallis are goof plants to try.
