usually the smallest tank you want for a quarantine is 20litres. And if you have gravel only use a very thin layer. The more gravel in the tank the more places for bacteria and parasites to live, and the more gunk can get trapped in it.
Having said that it could be used to hold a couple of small fish like neons for a few days if necessary but you will have to keep the feeding down and watch the water quality closely.
Wouldnt that make it a 10 gallon, not a 10 litre tank?
It would be close to a 10 gallon if those were inches and not cm. At about a foot long by 8 inches by 6 inches I am more than ready to believe it is only a 2 1/2 gallon tank (9 litres). As some have said it is about minimum for a betta tank if you can heat and filter it properly. It could also hold some of the smaller things we keep like heterandria formosa or shrimp. Again these would require adequate filtration and temperature control.