Hi and welcome
How many fish did you have in there? It's not a huge tank and it should be started off with just one or two fish until the tank has cycled. After that, the fish should be added slowly.
In case you are not familiar with it, cycling describes the process where good bacteria build up in the tank. These bacteria convert the baddies (waste products) into more harmless products. Even after that, small and reasonably frequent water changes are still advisable.
As Sharn says, look up cycling in this forum. There is heaps of information on it.
Don't give up, we all keep learning in this hobby.
Cheers
Jude