Welcome to the forum. First bigger honestly isn't better per say it's just easier. The more water volume the more time you have to correct any issues.
Sumps are not the be all to end all. There excellent for many reasons,but not necessarily needed.
Lighting these days is led coming on like...
Ok, the 20 is way to small. The stress of confinement isn't helping matters. I use a 29 for qt and I hate putting tangs in that. Luckily none have had issues to deal with in it
As far as coral,what type of lighting you have plays a huge role. Inverts you can do shrimp ,I don't recommend coral banded. These will also help as your cuc. A variety of snails is also helpful. Astrea,narssius,cerith,ect
Triggers are big eaters so that's normal, the twitching tho is concerning. I was thinking the same thing about qt quills, but haven't we all done it? Lol
Agreed. If you look the next time your at the store, most fish tanks are run off one system. Then inverts and low risk fish like draggonetts are often in another. Bc copper is in the fish system. By low risk fish I mean due to them being less susceptible to certain diseases.
A deep sand bed has benefits and down falls. It will trap nasty gases,and toxins deep in the bed that if ever stirred could wipe out the tank. However yes it will hold alot of fauna and help with controlling the nitrogen cycle.
Some folks use no sand and go bare bottom. Personally I like...