The newest newbie

[moniquebarclay]

when you say 15 L is that 15 Liters or 15 Gallon long style ? either way is very small for a shark

It's 15 liters.

 

[Tifftastic]

Ok, so 15 litre is much too small for any of those fish, with the exception of the betta (Siamese fighting fish). I would highly advise either upgrading your tank, or rehoming the shark and the neons. The red tail will need a tank that is about 270 litre (IMO) as he will get 6-8 inches long and rather wide, as well as fairly territorial. The neons really need to be in larger groups (minimum group size is debatable, IMO, but the general suggestion is six minumum) which will require that they be in a larger tank, somewhere around 90 litre.