Do I need to add chemicals to my fish tank?

I currently have a yellow goldfish, a small bubble eye goldfish, a small oranda and a very small algae eating shark. I have a good sized tank for the 4 of them. However, I am really struggling to keep the tank clean, I clean the filter every 2-3 weeks and the shark is supposed to eat the algae off the rocks (he... show more I currently have a yellow goldfish, a small bubble eye goldfish, a small oranda and a very small algae eating shark. I have a good sized tank for the 4 of them. However, I am really struggling to keep the tank clean, I clean the filter every 2-3 weeks and the shark is supposed to eat the algae off the rocks (he doesn't) and I have a water plant that should help keep the tank clean. The water is still going green very quickly (3 weeks max) and I need to remove my fish to put in fresh water. Every time I put in new water I must wait 24 hours for the chemicals to settle before i can put the fish back in. Is there any way I can fix this problem as I fear my fish are going to die due to being taken out of their tank and kept in a bowl of still water for 24 hours at least once a month.
Thank you for anyone who can help! It will be greatly appreciated!
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