[Sk8r]

Start withdrawing media from wet/dry, AFTER you have deepened the sand bed [use fine aragonite, funnel into half inch hose, 1 cup every 4 days]. WHen your sand bed has reached 4", withdraw one layer of media every 3-4 days. By then you should have a stable sandbed.

Establish a refugium to feed your mandarin: it will add additional sandbed, [4"] as well. Plus the cheato will remove all algae from display tank.

Use ro/di only.
I'm sorry to say the tangs really need to go with the trigger. Adult size on the hippo is eighteen inches. You can have a lot of varieties of fish that aren't that large, but there are 2 problems with the big tangs: they eat so much vegetation that they will increase phosphate and harm the corals chemically in that small a tank, especially with a smaller skimmer,---and physically, they just don't have room to grow and run as they need to do. You have what you need for lps or softies, but your fish are overwhelming the system. I'd recommend a 20g fuge.