Have been out of the reef hobby for about 5 - 6 years and will be starting small, a used 50 gallon with a separate sump. The system as it stands now has a plumbed overflow, plenum, wet-dry connected to a Little Giant 2MD-SC with a gravity type back flow valve. I have covered the old bottom return holes and ensured the tank is watertight and plan on removing the bio-balls from its wet-dry, maybe replacing them with some live or base rock. I will also add a SCWD Squid Wavemaker to add some turbulence. The existing protein skimmer is an Aqua C Remora, and to which I'm considering purchasing a skimmer to help surface movement and better protein removal/skimming.
I plan on using CaribSea special grade reef sand, about 80 pounds in two layers over a newly installed plenum with a fiberglass screen between the two layers. Also getting 80 to 100 pounds of decent live rock, preferably aquacultured, if possible. And will replace all my old test kits: ammonia, nitrite, nitrate, calcium, pH, dKH, phosphate (La Motte) and replace my ORP and pH probes that I had on my old system years ago.
Water will be RO with DI for silicate removal - SpectraPure equipment. Salt will be Instant Ocean or Reef Crystals. Salinity will be measured with a refractometer. Water changes will be 15% of tank and sump combined every two weeks. Heating will be two 150W submersible heaters and a chiller is available if needed. I plan on turning this into a mini reef with hard and soft corals as well as a few fish, 1 Tang (probably a Kole or Yellow Tang) 1 dwarf or pygmy angel, 3 green Chromis, some shrimp and brittlestars, etc.
Now for my big questions:
Lighting - I am contemplating T5 HO such as Aquarium Technik Innovation brand and over driving them with Icecap ballasts. Eight lights by ATI, six driven by Icecap ballasts. They will be 36" with individual reflectors and all will fit over the aquarium. Which would be a better choice, and what combination of bulbs would you recommend? AquaSun, Actinic White, Super Actinic? Alternatively do I need a MH set up with these actinic lamps?
Should I add gravel in the sump, and if so what type? Is there any benefit to adding refugium fauna to the sump if I put gravel in it? Will shrimp, worms, snails, brittlestars etc., be beneficial if added to the main tank? Is there any benefit to adding GARF's gunk to the tank assuming I can get it?
Do I need to test for anything other than the tests I've mentioned such as iodine, magnesium, or strontium? Are there any supplements you would recommend, i.e. to keep appropriate calcium dKH and pH? Should I be using carbon? I used to use Boyd's Chemi-pure in the past, and should I consider phosphate remover also?
Do I have enough water circulation? Should I add a powerhead or two? Have I missed anything important or obvious?
Welcome back to the hobby and thanks for your questions, as there are some areas where changes are recommended. First, your equipment looks like top of the line brands, and updating all test kits is an extremely good idea. As for installing the plenum in the main tank, that is no longer recommended as its bed requires more care than what can be given when in the main tank where it is usually mostly covered with live rock and corals. Even though I know of one plenum system that has been in the main tank for over 12 years and still functions perfectly, most in main tanks have experienced conditions no different than what deep beds experience when not cared for properly, e.g., system nutrients increasing and experiencing unwanted algae growths.
That's because the greater majority of plenum beds/their volume, need to be vacuumed monthly and checked for clumping sand, and if found, broken up. Without the bed remaining clean and free of clumps, no type sandbed will remain effective! (Would you never dust your home?) Therefore, consider installing the plenum in the sump if there is room, with no sandbed in the main system or nothing more than a 1 inch bed of crushed coral. Furthermore, the plenum dividing screen is no longer recommended 'unless' planning to have animals that will burrow deeply into the substrate. Of course, when in a sump, the bed remains wide open to proper maintenance. Therefore, sump or interconnected tanks are the perfect way to utilize plenums.
As for the sand grain size for the plenum 4-inch bed, recommend crushed coral (4 - 6mm), and if using CaribSea products, suggest using their product that comes from the Caribbean. They also have a product called crushed coral that comes from the US mainland, but the difference is the substrate from the Caribbean contains aragonite, whereas the other contains mostly calcite. And one does need to know where it comes from, as just saying its 'aragonite' is misleading, especially if not told where it comes from. The same substrate can be used in the main tank if your wish.
As to your question about placing gravel in a sump - all necessary biological functions 'will' be handled by the plenum sandbed or the small bed in the main aquarium and its live rock. Duplicating it elsewhere is not necessary, and besides, it would also serve as a collection device for detritus/nutrients.
Replacing the bio-balls in the trickle filter with a few pieces of live rock is a good idea when used in reef systems, as less live rock is then needed in the main system, which then could be limited to about 30 pounds of 'branching' rock if available. That way, the rock structures themselves allow better water flows, as some passes through the structure instead of being bounced off 'piles' of rock. It's simply better all around circulation in the aquarium, besides more secure places to place corals. Also, an overflow for the Remora skimmer is a good idea, as I've had one on a past system and it worked nicely.
As for a salt mix selection, of the two mentioned, my preference for a 'reef' aquarium would be Reef Crystals, as that will aid somewhat in maintaining the proper calcium, alkalinity, and magnesium levels for your corals. As to water changes, consider 5% every two weeks adequate.
Lighting is a subject with many possibilities, as product selections are vast, and need to be related to what species is to be maintained. From your description of things to come, I think T-5's (10 and 20K) are the way to go until the marketplace resolves the issues surrounding LED's, as I think those are the ideal lamps for aquariums. Keep in mind when overdriving the lamps, they wear out faster! And as to adding metal halides, I think you can accomplish most goals without them, as the T-5's should suffice nicely for most applications.
I've found in past systems that 'nature' is quick to establish various 'detrivors,' therefore; I have always allowed Mother Nature to take care of that aspect. As to additives, if the system is not overcrowded, water changes and a two-part calcium/buffer solution should take care of calcium, alkalinity and pH. Magnesium should be monitored and adjusted as needed, and there are many excellent brand Mg additives on the market. As to Iodine, minor additions here and there, with less than recommended on additive bottles are my thoughts on this element. The same is true for strontium for your planned bioload. And 'yes' you must use a quality phosphate remover, and right from the beginning. A small reactor or canister filter would be the ideal way to handle that aspect. If using a canister filter, activated carbon can be placed in that also, and I use various brands, e.g., Boyd's, ESV, and Warner Marine to mention just some.
You may not need additional water moving devices, but if so, small powerheads/magnetic mounted powerheads, some with swiveling nozzles (FLO) could resolve low current areas in the aquarium.
Hopefully you find the above helpful.