Just read your post with interest. Im glad to hear the clowns are swimming happily. I am afraid to say that you are higly likely to get real problems ahead of you. The tang is totally out of its environment for a tank this size. 100 gallon minimum for a yellow tang.. 125 gallons or more for most other tangs.
Tangs grow large and are open water swimmers. 10 gallons ( I assume this is US gallons which means its probably only 7 UK gallons) will stress the tang probably to the point of sending it mad or catchijng whitespot (tangs are easily stressed and whitespot is ususly what they bring with them)
In a 10 gallon tank you should be looking at a stocking level of 2 clowns.. perhaps a goby but nothing more. Marines cannot be stocked like freshwater fish, they give off enormous amounts of waste material when compared to the freshwater cousins and thus they needs more space per gallon. They are also highly territorial and a tank of this size is going to cause problems with the fish you have as they will eventually beginto fight over this space.
lastly.. you mentioned the tank was cycling for 2 hours and yet you had all these fish in the tank.. you have no sand or live rock and only gravel? If this is true then i give the tank a week before you get a wipeout.
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The ammonia is going to increase at a high rate (marines give of alot of ammonia, they even give out ammonia when they breath).
If you arent using live rock then you tank needs to cycle for about a month before its safe to introduce any form of fish. with the amount of fish you have in this tank the ammonia is going to go off like a rocket im afraid to say.
I would urge you to return the tang (preferrably all the fish) and get at least 10lbs of live rock, this will cycle the tank far faster. get a skimmer in place if possible for a tank thjis size as it can reduce waste before it gets changed into ammonia.
Sorry to sound so negative but your lfs has really done you no favours and the money you have spent out will be wasted in dead fish within a week or so. It makes me mad to see shops trying to make a fast buck with no regard for the welfare of the fish and any care for the customers that they should be trying to make return for more items at a later stage.
Hope it works out well for you.
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