20L build, for freshwater noob

I've never kept one, and I know they can be aggressive, but coloration wise a dwarf puffer would have looked good in there. It's too crowded now though. They can get territorial.

What type of gourami is it? Sometimes they can be aggressive, but not always. Sometimes getting two gouramis will take care of that problem. It helps to get a male and female, I've read, but I can rarely find females.
 
Yes i did get some established filter media, and i added some plants as well. I just tested it and it's still testing fine. The gourami calmed down, i guess it just needed to establish himself. I have been looking at figure 8 puffers, since they seem to be the best fit for my situation. Would it be okay to add one? and what to they prefer as far as food goes?
 
What type of gourami is it? Dwarf, Blue, Opaline, Kissing, Pearl, Honey?

No Fig. 8 Puffers, they need brackish water and a 30 gallon tank to themselves.
 
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