darn dog woke me up early
was hoping to sleep in.
the qt is the right size. it's actually a great size. all you need in that is a hob filter like an emperor or something, and a biowheel filter is even better. a maxijet 900 as well to help with movement, a heater and a flourescent light and that's all you need.
except since you put copper in there you need a copper test kit to see what your levels are since (supposedly) it can leach into the silicone.
i have a sponge prefilter that i soaked in my tank for months to get a bacterial colony, and placed it over the intake of the emporer filter. i've not seen ammonia in my tank even ghost feeding.
i implore you, before you get another tang, try some other fish. you seriously only have beginner fish in there (damsels and clowns). definitely get rid of the damsels if the lfs will take them back, and maybe try a fire fish, royal gramma or a dwarf angel, or something more hardy. acclimate it to your quarantine tank, let it lve a stress free life for a few weeks, then acclimate it to your DT.
i'm eventually going to get yelled at, but it's worked well for me for a bunch of fish (even an unhealthy kole) and others say it works for them. put the fish in a tupperware container with as much water that can fit in it from the bag. every 10-15 mins take a turkey baster of water out of it and discard, and take a turkey baster of tank water and put into the bowl. for hardy fish, do this for 1.5 hrs, for more delicate fish do for 2+ hrs. also, test the water from the lfs--if there is a difference in salinity and pH you may have to bump this time up and lower the amount of water you transfer. I've not lost a fish due to acclimating this way.
I was also thinking that your 10 fish in a 75 may be over the max capacity for the tank, and by throwing in a poop machine that is a tang you may have caused an ammonia spike that you may not have been able to read because you were more worried about the fish. definitely take them back.
it's good that you repositioned your pumps, and maybe you had some bad stuff going on back there that may have been the ultimate issue with your tank. i have 3 mj1200's in the tank and a mag5 in the sump, so your pumps should be fine--i have 2 mj's pointing towards the middle of the tank from either corner and the third going around the back of the tank.
a temp swing may also have been screwing up things as well so its good that you raised the temp. just realize that now your tank has a higher base temperature for it to get hotter from and you may still have a temperature fluxuation.