Okay, the filter we are talking about is the Regent/ Aquatec, right? if so, then this is what is known as a powerfilter or HOB filter (hang on back). A canister filter is a waterproof module that is separate from the tank. Only hoses run from the tank to the pump.
Okay, for a properly filtered 30 gallon aquarium, with two goldfish that are growing, the total flowrate should be anywhere from 275 to 350 gallons per hour. this pertains to HOB filters and internal filters ONLY. Canisters filters can have a much slower flowrate because of the excessive amount of space (bio-media) they have for beneficial bacteria to grow. Plus, the water is FORCED through the bio-media. That makes canisters the best.
Anyway, the flowrate of your Regent/Aquatec filter is only about 150 gph. 170 tops. Anyway, to have sufficient filtration for your tank, you need about double what the single HOB filter is providing. Now, If I remember correctly, you also have a small internal filter running right? Well this wont count for much if there isn't a substantial amount of area for beneficial bacters to colonize (bio-media like a large sponge). If it does have a nice sized sponge, it will help but it wont make your tank sufficient to keep up with your goldies.
So, That is why I suggest getting another filter equal to your one with the bio-wheel.
Now, you will need to keep doing waterchanges every other day or so until your water params go to 0ppm for ammonia and 0ppm for nitrItes and they STAY there. If they DO go to 0ppm before you get your added filtration, this does not mean it will remain that way if you skip on the added filter. As I said before, they are still growing......
Hope this helps!
Paul