There are some options but not a lot of them for such tiny space.
1. Add 5 danio zebras. There are crazy fish and make any tank look lively. They are always happy and do not require much upkeep: they pick food from everywhere - surface or bottom.
2. Plant the tank and let some tiny irridesent fishies (do not know their names). It will be decorative. Plants will require attention though.
3. Put 2 dwarf puffers and get some drama going and watch they smiley faces looking at you through the glass begging for worms.
4. FInal drastic measure: go bold and buy 2 tiny baby oscars (i saw them, they are adorable *thumbsups ). Soon they will grow up and you have to buy 30g for them, then 55g and then 125g. So, in about 3 years you'll have at least 3 tanks plus QT tank and hobby going full strength.
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