Before adding anything, you have a very strange and unsuitable mix of fish in that tank. I'd spend time sorting this out before you even considering adding more fish. For example:
Goldfish - luckily, yours is in a decent-sized tank, but they're not tropical like the other fish you have. 30g is the minimum for a SINGLE goldfish, not 1 goldfish and more fish on top of that.
Knife fish - reach a metre, and aren't suitable for the vast majority of home aquaria. As already mentioned, your other fish will be eaten as it grows. Other fish might be safe now, but wait til he grows.
My suggestion is buy a much bigger tank for the goldfish, without heating and with filtration. How many other goldfish do you have? I'd also suggest you returning your knife-fish to the store asap.
To make it easier for the rest of us to help, could you possibly post a list of all the fish you have, i.e numbers and rough size estimates?