Why in the world would they sell you a freshwater kit??? Yes it makes a difference. You really NEED to find another fish store, they are selling you junk that you don't need, and fish before you even cycled the tank. Saltwater kits are geared for SALTWATER and it's readings, they are not the same.
Also API reads nitrates high...like 4Xs higher then it actually is. So stay away from API and Redsea kits, both have bad reputations. Go on-line and purchase a saltwater test kit, master kits are cheaper then trying to buy them all seperate...Seachem, or Salifert I know are good brands. You can purchase SALTWATER ammonia test strips, that way checking ammonia takes seconds. ONLY ammonia, you need the lab drop type kits for everything else.
PH
Alkalinity or DKH
Calcium
Nitrites
Nitrates
Phosphates
Ammonia
The only thing you need to worry about right now is ammonia.
As for taking care of the extra food...a CUC (clean up crew) is what will take care of that. In saltwater the cleaners actually do a great job. Serpent or brittle stars, a variety of snails...Turbo, nassarius, nerite, and Cerith to name a few. There are cleaners in the live rock as well, bristle worms, bivalves, sea slugs, and tiny micro-stars. The uneaten food that sinks deep into the rocks, is not being gotten out by the HOB, that's the CUCs job.
The different CUC critters feed on different types of algae, some feed on uneaten food and dead stuff. I don't recommend hermit crabs, they kill snails, and it gets expensive to replace them all the time.