yes you can
A sandy bottom shouldn't bother triops. In fact triops like sand in the bottom of the tank because they can lay their eggs in the sand and cover them up. This prevents other triops from eating the eggs, and lets you hatch another batch later. Just make sure you wash the sand first, and don't use ocean sand because it contains salt that could mess up the pH for your triops. This link has more information on what kind of sand triops like and how much to add : http://www.triopsguys.com/triop-tank-setup-for-t-australiensis-21/
Its the rock, sand, gravel, whatever on the bottom of an aquarium.
plecostamus
Moldy cheese or an eyeball
Anything labeled safe for aquarium use. Beyond that, it depends on what type of fish you are keeping, and whether or not you have plants, for which type of aquarium substrate (gravel or other lining for the bottom) should be used.
I would say yes it would be ok not to put anything at the bottom of the aquarium. Gravel, stone and plastics are usually used for appearance. If it is a freshwater aquarium and if you plan on using live plants then gravel is necessary.If it is a salt water aquarium then it is necessary to use live rock and live stone.
No! :)No freshwater aquarium fish to my knowledge is poisonous.
They can just live in an aquarium with the normal pebbles on the bottom that all fish live in.
No, seahorses do not bury themselves. Chances are they have died and have been consumed by something in your aquarium. If there is rock it is possible they are hiding in the nooks and crannies. However, many pipefish species are difficult to maintain, so in all likelihood they have perished.
Refraction - bending of lights.
You can find Kenta in Aqua Road. Head into the Aquarium and one of the portals will bring you to the Aquarium Zoo. You can find Kenta at the bottom of the zoo map.
It will seem lethargic and may look redder or grayer than normal. It'll also not swim around much and may lay on the bottom upside down.