What to do with a 20g tank

I have a spare 20g tank that I am looking to use for something. Does anyone have an idea of what I could use it for that doesnt require the amount of time and maintenance that my reef tank requires? I would like to keep the setup as simple as possible. Maybe just a canister filter, light, and heater type of thing. I already have another tank that I use as a quarentine tank so I dont need one of those. I dont want to spend lots of money on this as the reef tank takes pretty much all my money, that and I dont have much being unemployed and all. I have the basic things needed for a tank but dont know what to do. I really dont want to set it up as a freshwater tank either. But if I had to I guess I could and maybe go with live plants. Looking for any suggestions you may have.

 

Make an invert tank with 20lbs of LR

 

frogfish. they are out of the ordinary and wierd looking. they dont move much but look cool.

 

or a clown tank with a med size green carpet

 

All good ideas, something to think about and research. Would I need a sump for those and a protein skimmer? I have my old CSS skimmer I could use but I really want as simple as possible. Doing further reading, could I keep a mantis shrimp in one with just a canister filter small light and heater?

 

that is possible. the frogfish or mantis may need a protein skimmer. if it were me i would just use the canister filter and get a HOB filter for chaeto and a mangrove. i dont have money for a protein skimmer so that is what im doing for my 29 gal and 10 gal when i have the money.

 

I've been keeping a small pair of true percs in my spare 20g. All I've got running is a canister and maybe 8lbs live rock. Kinda cool and also kinda boring. If I had a little xtra $'s I'd get a t5 light and put in some encrusting corals. Anything to add a little more life I guess