It would be a crowded tank. Depending on type of shark (bala, red tail, rainbow, etc.) would depict the amount of time it would work. However no type would be very long lasted.
If you have a red tailed black shark that is in a small aquarium but you now need a bigger one, you can buy a cheap one through Amazon's UK website. You will be able to find a 50-gallon tank for him to go in.
The choice is entirely yours. Either fish would be OK in a small 10 gallon tank provided it is heated and filtered properly.
Yes. I have seen it in tanks as small as a 10 gallon tank.
Not a lot. A twenty gallon aquarium is too small for a 5" fish to begin with.
A small species of sunfish would likely be alright in a 50 gallon, however many species of sunfish get quite large, on the order of 8 to 10 inches long, and would be too large for a 50 gallon aquarium.
A ten gallon aquarium is definitely too small for piranha. Piranha grow to about 8 inches in length, can be active and jumpey swimmers, and produce a lot of waste. All of these factors make it not suitable to keep them in anything less than a 90 gallon aquarium.
A one gallon aquarium is very difficult to maintain, however it can be done with proper filtration. An under gravel filter however does not work for saltwater very well at all. If you only use large substrate then it can work with constant water changes. I have a one gallon aquarium for a bright green mantis shrimp that I run with a small internal filter. I take a small cup of water out of it and replace it with my hundred gallon every once in a while. Tank has been up and running for almost a year now with no problem.
For twenty small (1-1.5 inch) Goldfish you should have something like a 30 gallon aquarium.
You can contain your 2 inch Albino Tiger Oscar in a 35 gallon aquarium with a 2 inch Pleco and 2 small Pictus Catfish for 4 hours.
A 29 gallon aquarium is far too small for a stingray for all but the most temporary of housing arrangements. So, no.
Freshwater aquatic snails would be the best choice for a tank that small.
The African dwarf frog can indeed live with fish in a five gallon aquarium. BUT BEWARE!!! dwarf frogs may eat your fish small enough to fit in it's mouth. so i recommend Platy and live-bearers as companions.