[Birdlady]

Welcome to 3 Reef!

While I cannot answer all your questions myself, I can get you started

General rule of thumb is about 20times turnover flow in your system. There are different ways to achieve this. You will count your overflow gallons/hour and powerhead gallons per hour. Example, if your overflow is 800 gallons per hour, and you have two powerheads each at 600 gallons per hour, then the turnover rate would be 2000 gallons per hour, or 20 times your system volume (approximately). That is generally speaking, for a softie or LPS system. If you are going for an SPS dominated tank, I think it gets bumped up to about 40 times flow.

From your pics, it looks like the gravel has buildup on it. Is it accumulated debris? or a growth such as cyanobacteria or diatoms? It indicates poor flow in your system now (which you mentioned losing a powerhead in there)

I have two tunze powerheads at about 600 gph each and they are aimed towards the front center of the tank from each back corner.
The flow hits the front glass and deflects, creating a more random flow. For a 110, I would put one at each end and one in the center, or, if you have a high powered return pump, you could split that into two or three outlets for a neater appearance.

Personally, I use no mechanical filtration at all. I do skim heavily. I occasionally run carbon as well.

Sump size, largest you can accomodate! In it, you could put your skimmer, heaters, return pumps, probes, and any other unsightly gadgets you may add to your setup. You could also add a refugium section to your sump (see the refugium section for threads relating to that)

If you add a refugium (or fuge), then that would impact the flow rate through the sump. Slower if you want one, and faster if you do not.

Hope this is enough to get you started! Please ask away!