40 gal seems to be important I've seen it stated online a lot and in a book I have on reef tanks for beginners. Get a book though I got the salt water for dummies it's a decent book to start with but doesn't quite explain everything just a good starter I got mine on the app market for android so I read it with my phone a few times.
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Cool, I set up a 10g after I had kept a 55g for a year or so just to keep corals too. Mainly because I have a puffer in the 55 and I wanted a peppermint shrimp I have a clown goby In the 10g too. I have to say I spend more time monitoring the ten but it's worth it
The easiest way to make the 90 a reef is get reef capable lights and put in corals. For the 20 check out taotronics lights, lots of people seem to be getting them these days and I've yet to hear something bad about them