So at this point I feel I have set my tank up incorrectly and i’m looking for some help, I recently got some constructive criticism and I really want to try to make my tank the best it can be, so I’m gonna be completely honest with my set up and any help on what I should do next would be awesome, I have a 20 gallon tank a nem who is dying, and creatures die about every month, I have four coral frags. I don’t have an ATO. I don’t have a canister filter. I have a hang on back filter rated for 30 gallons. I have a heater and a powerhead. In my tank, I have a clown fish. A royal Gramma a tail spot blenny and a yellow watchman goby I also have five Trocus snails, 3 nasarius snails and a turbo snail, I have around seven blue legged hermit crabs I also have a pom-pom crab, a porcelain crab, and a pistol shrimp, and a feather duster I had that went missing. Any help would be greatly appreciated on what to do next

