Certain types of fish, such as catfish and plecos, are known for cleaning the bottom of a tank by consuming algae and leftover food.
I suggest that you learn more about your fish in your aquarium. One of your fish might be a bottom sucker which means that they eat all of the dirt on the side or bottom of your tank. If one of your fish are bottom suckers, that's probably the reason why your aquarium is clean. If none of your fish are bottom suckers, than somebody must be secretly cleaning your tank for you.
Halibut, Sanddab Flounder
Take them out and add what you forgot most of the time is water solution
Some types of fish known for eating brown algae in aquariums are Siamese algae eaters, otocinclus catfish, and bristlenose plecos.
The bottom fin on a fish acts like a keel on a boat. It helps keep the fish right side up in the water.
Some types of fish known to make sounds underwater include croakers, grunts, and toadfish. These fish produce sounds by vibrating their swim bladders or rubbing their body parts together.
Various types of fish eat clams, including species like flatfish, scup, and some types of drum fish. Predatory fish such as flounder and sea bass are also known to feed on clams. These fish typically use their strong jaws to break open the hard shells of clams to access the soft flesh inside. Additionally, some bottom-feeding fish may scavenge clams as part of their diet.
Different types of fish swim in different layers of the water. Top floaters, bottom dwellers and middle swimmers.
Some types of fish that are known to eat brown algae in aquariums include Siamese algae eaters, otocinclus catfish, and certain species of plecos.
In a Bowl!
Most fish have only two heart chambers.Amphibians and some types of primitive fish have four heart chambers, and the fourth one of these is known as the conus arteriosus.
http://takemefishing.org/fishing/fishopedia/preparing-your-catch/cleaning-your-fish/