This question is best answered by the fish.
Many fish have sound reasons why they would prefer one form of substrate over another.
Some fish need to burry in a soft sand due to several reasons , some use this burrowing to await prey some to hide during daylight ,and some just find it easyer to feed in soft sand.
Some fish need fine gravel for the same reasons apart from hiding in, they have mouths developed to “Pick” out food or plant matter.
Many fish have developed spawning habits that need a certain substrate or decoration.
My self before setting up a tank I read and check all information on the fish I wish to keep / breed and then I go about setting up the tank to fit the needs of the fish.
I am lucky in that I can have several tanks but for my main tank in my house I use fine gravel.
You also need to think about plants some plants can’t grow in sand so a pot with the plant placed in along with a media that will assist growth can be a solution.
The main problem with sand is it will pack and allow the growth of bacteria, having said this keeping the sand as a thin layer or disturbing it at regular opportunities is a solution.
Ok now let me explain about poo in my tanks.
I have never vacuumed or removed Poo or any waste for that matter dead leaves, plant matter, old food, dead snails or any thing.
I breed mainly Hypans and would not say I am a very experienced or successful breeder but I have a few Species/ variants / L Numbers under my belt.
I always have quite a lot of mulm in my tanks; I do however have massive filtration ie an Eheim 2128 on a 2 foot tank an Eheim 2324 on a 18” tank. Along with a power head with a large foam filter pad that I change weekly. This seems to break down the waste.
I empty my tanks around every 3 months to remove all the fry and move them to grow on tanks.
I do not believe poo is a problem or needs to be cleaned up removed. Once the Ammonia and other components have been “neutralised” by the nitrification process of the filters its only harmless gunk in my opinion.
I only clean my filters once a year and that id to remove the sand or small gravel from them, the media baskets get a rinse out in a bucket of tank water and the foams get changed for new ones.
I do believe if you get a balance right you don’t have to clean up or vacuum in any way.
Small fish loads ie 2 foot tank with 6 x 2” Hypans at the most, feed once a week and daily when you have fry. 10% water changes a day using RO and HMA mix.
I have never had any problems and I prefer to concentrate on getting the tank to mimic nature.
