How long is the tank? The common opinion (I think) is that some tangs (like smaller bristletooth tangs) need a minimum of 4 horizontal feet as they are constantly on the move. Yellows, purples, sailfin (zebrasoma species) need about 5 feet (maybe the butterfly too) and your larger hippos, vlamingi and Nasos need about 6 feet. But, from what you’ve said your tang and butterfly are babies. Watch them as they grow. Your best indicator of overstocking would be as
@nereefpat said. As long as the are happy (not fighting) and your tank can handle the bioload, you should be fine. Sometimes a fish can get sick because of stress from overcrowding. Most fish harbor diseases/bacteria (as we do) but only get sick when their resistance is low.
However, I would be remiss if I didn’t mention filtration and water movement. If you are using canister filters, clean them weekly. I’ve seen many reef tanks run with canister filters but they require diligence. If not cleaned regularly, they can become nitrate and phosphate factories. Keep them clean. You also need to consider water movement. You indicated you have a power head/wave maker. What brand is it? Tangs, IMO, need lots of flow. They love to play in the stream of a power head. At least mine do. A power head also helps keep detritus in the water column where it can be sucked into the canister filters. If you do anything else, buy another power head.
If there is a public aquarium in your area, I would recommend you go there to see if they have a large tank with full grown tangs (or nearly so). Eight inches of full grown yellow tang is a lot larger than one would think. First off, The typical measure of 8 inches doesn’t include the tail. Secondly, full grown tangs are thick fish. As they mature, they bulk up and mature. They end up looking waaaay bigger than 8 inches. Eight inches is a big fish.
I’m not criticizing your choice of fish. I have a small yellow tang in a smaller tank than I think is appropriate. But, I intend to rehome him when gets larger. I like watching them grow. Mine is currently only about 2 1/4 inches from snout to base of tail, but when he reaches 4 inches or so, I will find him some place else to live.
Go slow when adding fish. Maybe add one fish per month unless it is a pair or small school (like three+ chromis or a pair of clowns). That way your tank can catch up with the increased bioload. Do frequent water changes, perhaps when you clean the filters. Most would consider my tank heavily stocked/overstocked but I do 20% water changes every other week, test weekly, watch my fishes’ behavior, and understand some of them will eventually get too large. It can be done certainly. Just be careful and deliberate. Research, think, research some more. Quarantine, watch and then watch some more. Ask ANY questions you have. We’ll try to help in any way we can.
