I kept my yellow eye tang in a 55 just fine. The yellow eye tang is one of the few tangs that can go into a smaller tank than most. A 75 gallon is more Ideal for a yellow eye tang, but I have seen worse than a 55 for them. Heck some people put Yellow Tangs in their 29 gallon tanks. (Which is just a very bad Idea!) I should note that I moved both my yellow eye tang and my yellow tang to my 120 from my 55. I knew I was upgrading so I bought both before the upgrade. Tangs are just hard fish in general to keep because of ICH outbreaks for random reasons. My yellow eye got ICH and died in his 120 gallon home for no reason! My Yellow Tang is doing awesome thou. So go figure! Had them both in my 120 for 4 months with no issue and out of no where the yellow eye tang died.
Honestly with that Lion fish in your tank and plus what you already have, I would say your stocked. You need a skimmer as lion fish really make a huge mess in your tank. Get a good skimmer!
The Reef Octopus BH-1000 is what I have and I would recommend that for your tank.
I would honestly slow down a little and wait a few months (4 at the lest!) and see how the fish are doing that you already have. You should be ready for more after. Honestly the Lion should have been the last fish in, being anything you put into your tank now will be considered his food.
Lions tend to not eat pre-existing tank mates, but new tank mates are fair game to them.
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