Interesting ideas, running a 2.5 gallon refugium on my 20 long i would have to agree, nano tanks are easy with a refugium, mine doesn't have any other filter system and as a result I don't really have to do 25 percent water changes, once a month, the cheato from the fuge cleans the water, and creates a haven for tons of bugs, hang on back the bigger the better as it will also increase your overall tank size , say 7 gallon with a 2.5 now you've got a 9.5, never ending supply of bugs, filter that can help you control the stability of a small system, and increase your bio load while doing less water changes and maintenance, can't see running a tank with out one now that i have one,night and day from before. My only regret is that I didn't buy the 5 gallon refugium
, next time I guess.