What about a flame hawk fish and 2 clownfish? Would that fit?
canister or hang on back filters are not "designed" for marine but freshwater aquariums, anyway you can use it or almost anything that moves water to make it useful if you make some modifications, I used to have a 20 gallons full of triving marine life with almost anything but 2 water circulation pumps and a radion g3 light, but it took me 4 years of small problems and all kind of equipment passing downthere in the sump till I decided to start removing one by one and see how it goes, I think that maturity in the marine aquarium is more important than almost anything else, but im beating around the bush, my advice: not to much sand, around 2 inches downthere, try to drill the tank and use a 5 or 10g sump so you send the water there and back up with a pump, and you use the sump to put whatever you decide, for 20gallons and a small fish you dont really need an skimmer, you need to let it cycle until your nitrates are under control, and get some good live rock for that, dont go crazy trying to add 1 or 2lb per gallon as everyone says, just put 2 big and porous ones around and wait, you can always pee on the water if you wanna do it to add a carbon source and make the cycle go faster but its up to you [emoji12]
But I do have a 55g tank I've been using for my goldfish. (I had 1 goldfish in a 5g tank when I was in my dorm but he got way too big and sick so I upgraded to the 55g when I moved out of the dorm and added a few more goldfish bc I thought he was lonely... he's alive still and happy in the 55g.) maybe eventually when that tank is empty I'll use it for a marine tank and get all the fancy stuff (granted I know 55 is barely in the fancy range of marine tanks)
I meant 10 deep. Whoops. I think it was for lizards or something. I found I'm my pops' garage
