If they're under the sand, they're nassarius. Ceriths are black/gray and stick to the glass.
You may have dead nassarius under-sand, which may be a problem but do not stir the sand! it can get worse.
Don't try Amquel until we've exhausted other methods.
Can you fit a Remora skimmer onto your 24g? That would do well for you, and they're an Aqua C, which is a good brand. Great, on what you're doing with fans and returns. The higher temp just means the water carries less oxygen, and makes the effects of ammonia worse for living things, but don't let it fall below 78.
You're asking the world's worst chemist about what ammonia turns into: it has to do with an uncompleted nitrate cycle, but there's no quick fix, because one of the things that may be at issue is your sponges and filtration system. Sponges and filter media are usually not recommended for reefs because they stall out the denitrification process somewhere around the nitrate level, and only cleaning the sponge gets rid of it. A clean sponge won't work, because the only denitrification agents you have, as I get it, is [besides your live rock and sand] the bacteria in your sponges, and removing them suddenly would make matters way worse than they are. Ideally you need 24 lbs of live rock, 24 lbs of sand, and a skimmer, and then you wouldn't need any of the filtration, or need to clean anything, but as you say, the skimmer is the issue---what you can get to go with that: must fit and must be active enough to do some good. If you're going to keep a packed coral tank, that would be the way to go, plus lots of bristleworms to help break down detritus---a handful of those guys can strip down a dead snail to a clean shell in 4 hours, they can get into any space, however small, and their poo isn't nearly as ammonia-productive as rotting snail is.
Keeping my fingers crossed for you. Like you, I wonder what the ammonia is turned into, or how much of the de-ammonia stuff could be uptaken by corals. I'm just no chemist and hesitate to recommend it at all, in favor of just seeing if we can get the tank's natural defenses to take care of this. Cleaning those sponges often and getting detritus out of them has got to help.
