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.
It's benefits are listed as :- Attractive rounded corners for safer handling Various fun and Exiting color (sic)
but key is :- Item location: 852, Hong Kong
Unfortunately the far east has yet to fully embrace animal welfare and fish in particular and treated horribly. It's a culture thing, while many of the inhabitants are beginning to see the light habit and tradition permit the continuation of cruel and outdated practices. This is the country that brought us the goldfish keyrings.
Basically: Sometimes they do, but most of the time they're too stressed. Juveniles that aren't comfortable enough in their surroundings to behave normally. They're also normally massively overstocked in a lfs tank because its a storage unit not a home. This overstocking makes them feel too vulnerable to attack.
Puffers don't eat fish so they don't attack for food but they do like their own space. In a very large tank you could mix a puffer with smaller species, this tank may be ok for them depending on the species of puffer, the personality of the ones you have and the aquascaping. Unfortunately the first sign of trouble is normally shredded fish floating around. :(
If you've been using an ammonia locking type product then these ammonia alerts are more useful. As they only tell you the levels of harmful ammonia and can ignore all the ammonia that is safely locked away. Regular test kits can't do this.
I've heard from several people who have used them with success, and a couple who had faulty disks. Things to note would be that they only measure TOXIC ammonoia, most test kits measure total ammonia. Toxic ammonia is normally only a very small percentage of the total but its the bit that you really have to worry about.
I wouldn't rely on it as a replacement to normal water tests, but it does offer peace of mind.
It very much depends. The water in a 400L tank will weight 400kg or 880lbs. When you add the glass, gravel, lighting etc you are looking at over half a ton of weight. Whatever it is on it will need to be very sturdy - not just for the fish and cost sake but also because if it falls it could easily kill anyone caught in its path!
I built a unit out of three 400mm kitchen units, and 2" square posts for a 450L tank, however it was wedges and bolted between two concrete walls so had nowhere to go.
You can custom built cabinets or get them made for you from specialists at not absurd prices - probably worth considering.
also not an expert but I know some people do keep LPS corals under T5. I'd recommend more tubes though. I had 4 full length T5 tubes and it was not enough. 6-8 perhaps.
Bearing in mind that tubes have to be replaced at least every 12-24 months and there will be a lot of heat generated you may find that a decent Halide unit give you lower long term costs and better performance.