Substrate in water?

[Oliviet#1]

How come people say that you have to replace the water when sand/EE get in it? I think I heard it was because it raises ammonia, but their ocean wildlife habitat is full of sand! What’s the difference? Thanks!

[Motörcrab]

In the ocean is a complete ecosystem with millions of plants and animals that eat and absorb all that cleans everything. In the crab tank all that stuff is missing to clean it. Think of it like swimming in a lake compared to a swimming pool. Most lakes are generally clean with water coming in and out. Pools are generally nice as long as they are treated. After a few weeks without treatment you wouldn't want to swim in it. Plus who would want to swim in their own poop!

[PinchersAndShells]

Branching off of what Motörcrab said, the ocean contains beneficial bacteria that converts ammonia (toxic) to nitrite (also toxic), and then converts that nitrite to nitrate (only toxic in large amounts). In aquariums, you have to let the tank "cycle" for at least a month in order to build up this bacteria in the substrate and filter, to prevent poisoning your fish. In the crab tank, we typically don't use filters since we provide such a small quantity of water, which prevents the ammonia and nitrite oxidizing bacteria from growing. So in short, it's a good idea to clean the dishes because the water bowls do not contain what is needed to prevent the build up of toxic substances.

[wodesorel]

Perfectly clean sand would have little to no effect on water quality in the short term since it's an inert rock. The water shouldn't go bad any faster than it would just sitting out. However, sand is easily mixed with bits of poop and food which will cause problems. Ecoearth being organic (bits of coconut) will start to rot quickly when in water.