My goldfish died D: HELP!?

well i have three goldfish in a 19 litre tank with filter and i heard thats not big enough. so you having 5 fish in a 22 litre tank is definately too cramped for 5 fish. especially as they are all messy and stuff. were they all getting a share of the food? and yeah dont put real life plants in there , only artificial, well im no expert but i dont know if youre alowed to put real life plants in there with them.