All those fish in a 30g does explain alot though. You probably just overloaded your bacteria with fish waste and what not. The tang needed a 180 or at least a 6 foot tank so it's no surprise about his passing, sorry......Tangs get sick and stressed in small tanks, even if they are little ones. Just the nature of them.
I would have MAX 4 or 5 maybe fish in a 30g. In my first sw
tank, a straight 29g I had four fish and they were ok in there. I also had a anenome that i returned because of the stress it put me through, they can actually eat fish and god forbid if they die, they can nuke a whole tank.
Since you have a 4 bulb fixture your first course of action should be getting new bulbs appropriate for saltwater. i would suggest 2 10k or 2 12k bulbs and 2 actinics (blues). Gives great color to the tank and can sustain those nems hopefully.
If you have an empty 120g and want a wider selection fish now is the time to do it right.
Is it a 6 foot 120 or a 4 footer? If you have a six foot tank you can eventually replace your tang. Plus you can add some really spectacular fish.
But if all you have the budget for is the 30g then we can heklp you work with that. if you could pick up a testing kit today that would be awesome, and just post the testing results. It will reveal ALOT!
Are you using ro
water? And I think i saw that you didnt want corals, is that correct?
You can setup a fish only 120g alot cheaper than a coral 120g for sure, as the lights price will kill ya! lol