You NEED to get rid of the shark, it should grow to at least 6" long and needs a tank 200l+
What I was saying is, is that you also either need more corys, or to take it back to the shop.
Here are some ideas:
Get a filter designed for a slightly larger tank - if you simply add 3 more corys then with your current stocking any filter designed for a 60l may begin to struggle so you need a bigger one.
Re-home the 2 platys - This will make room for 3 new corys to be added to the tank.
Re-home the lone cory.
Basically your tank is classed as 'fully stocked' at the moment as when the fish reach maturity the bacteria in the filter which cycle toxic ammonia and nitrite will be able to cope with the levels produced.
However if you add more fish then there wont be enough room for the bacteria to grow, and so ammonia will start building up and kill the fish.
There are only two ways around this:
A) Buy a filter designed for a slightly larger tank
B) Add NO more fish, but of course that would be unfair on the lone cory.