Tetra 25775 Whisper is rated at "up to 60 gallon" if I remember right. Usually we choose one step up when it comes to picking a filter for aquarium. You can never have too much filtration. But it is fine for now if you don't have ammonia build up. Goldfish will grow to fairly large in size, especially when they are common goldfish. Their bioload will increase as they grow larger. In any case, it is a good idea to have a water test kit for monitoring ammonia, nitrite, and nitrate.
I would recommend you to do minimal 30% partial water change during weekly aquarium maintenance. We typically do 30~50%. Your 15% is on the low side. It is to prevent the build up of nitrate and other organic waste.
Always use a water bucket to prepare the water. Let it sit for a day or two to have the same room temperature as the water in the fish tank. So it won't shock the fish with sudden change in temperature.
Yes, a lot of people had the false impression that fish can be picked up and brought home any time you decide to have them. It is why majority of the fish don't live more than a few weeks after people bought them. The aquarium always needs to be set up properly before we can bring fish home.
But it is fine for now if you don't have ammonia build up. Goldfish will grow to fairly large in size, especially when they are common goldfish. Their bioload will increase as they grow larger. In any case, it is a good idea to have a 
