can't see what it is exactly from pics. if it's gravel there'll have been a build up over the months since you started and you say you've had no snails of any length of time so it sounds like you've got nothing really getting stuck into the detritus etc and you can't rely on getting it out yourself. i also personally wouldn't use any replica rock only live rock just my opinion tho.
Most hermit crabs will eat small snails and take the shells. Personally I'd remove the crabs if possible and use more snails.
Most common cause of nitrate is from over feeding or over stocking. Next would be a problem with the water purification filters. If you're buying your water ready salted there could be just about anything in it. Even the best lfs can make a mistake and a badly fitted or degraded o-ring in the RO filter is all it takes to end up with a bucket of tap water (with high nitrates) instead of purified "RO water".
Ask the LFS to sell you pure, unsalted RO water and ask them to show you the TDS before you pay. TDS is Total Disolved Solids and it should be zero in pure water. If they only filter with an RO membrane and don't then use DI resin the TDS will be anything up to 20 which is ok, but not good.
If they don't know what TDS is then they have no idea how to filter water and you should avoid them! You can buy a TDS meter for under ?20.
Once you've got your purified water it's easy to add your own salt, bubble the water with an airpump to mix and aerate and then test with a refractometer.