I keep fishes, brittle stars, and corals with my mantis. You can feed it frozen cubes of clams, squid, and Formula One from Ocean Nutrition. Ocean Nutrition seems more dense than the watery Hikari.
As the crabby guy I'd say they didn't starve to death. Some just die either naturally or get murdered by the others. I feed by critters as they are part of my main display in that tank. Sinking pellets are fine twice or thrice a week if you are not feeding any other fish.
I think it's the rocks...either the LR or base. I've used Bio-Spira on freshwater and it works. But you don't need it for this time as you already got the stuff.
You can strip out the lights and ballast for more wattage. This allows you to keep the hood intact.
Why would you want to get rid of the lid? Without the lid, you need to have a canopy custom made.
Iodine overdose can kill fishes and critters. Happen to me once. That is why dipping corals in an iodine solution will make all the critters come out an die.
The flat worms don't move around too much too. If you have lots of them and pay attention there will be a few that will be moving fast while the rest will just sit still.
Disposable camera doesn't work to well beyond a few feet...and I mean a just a "few".
Olympus and Canons have a lot of inexpensive digitals with underwater (NOT all weather) housings. I've never used an Olympus but have used several Canon housing in the past few years...
Last I heard a few years ago, you can pickup corals that are attached to rock rubble thats no bigger than a quarter. However, that was years ago so you have to check.
Spong needs to be clean every few days. And for the amount of work...doesn't help too much. I could use that space for something more important...like water.