Smallest tank that can be reasonably cared for?

[FloridaMicroReef]

For me, my tiny fountain pump raised the temperature of my three gallon tank up to 82 degrees. But I live in Florida, and the ambient temperature of the house was already 78-79 degrees. If I lived where it was colder, this heat gain would be an advantage.

It was a cheap fountain pump.

 

[58e970b2-3f88-4897-87ba-5]

I have a 2 gallon jar that ended up with a 0.5 - 1 gallon sump (never measured the sump size). At this time I don't think I would go smaller, since I was having issues with 100% water changes.
Coral seems unhappy if I do a full change so I just try to change the volume of the sump every week and use a filter sock for a few hours if I get food settling on the bottom (maybe once a month).
I am slowly setting up another 2 gallon without sump. If I can get the 100% water change system figured out I may be interested in trying smaller.

 

[danieyella]

I have a little 2.5-3 that's going pretty smooth so far. I do approximately 90% water changes once a week. IDK if I'd go smaller personally.