2.7 gallons is too small for fish. Look up minimum recommended tank size on the livestock you wish to keep before building a tank. This mindset is backwards of what we should be doing. We are building life support systems for livestock. So we need to figure out what we want to keep, and build a tank around those parameters. Instead, we build a tank and think "ok, now what can I put in it?" This seems to happen more often with nano tanks. I guess the all-in-one setups make it that much easier. I have yet to find a fish that has a recommended tank size under 5 gallons.
This isn't rocket surgery