Aquaclear spitting micro bubbles and fuge questions

[Koresample]

I have a 1 month old AC 30 PH that i use for circulation and i have noticed it is randomly spitting out (big) clouds of fine bubbles, is this a defect with the impeller or something else? I also have a question about adding a refugium. I dont have the space for a sump, so i would have to hang one on the back. I have seen several articles where people have made there own from a HOB filter, like an AC etc. What is the purpose of the fuge, and what do you stock it with? I seem to have a ton of copepods and amphipods in my tank (fallow now except for my snails, hermits, LTA and some shrooms due to an Ich outbreak), do they naturally go in there and multiply or do you stock it with something that will grow and provide nutrients? I want to add a mandarin but have heard they need a ton of these to feed on (seems like i am well on my way there) but how else do they multiply without running out 9my tank is only a 40 g and i have read that one mandarin should be in at least a 75g tank).