Substrate is one of the biggest potential issues beginner goldfish keepers have. Substrate can harbor toxic chemicals, dangerous bacteria, trap uneaten food, and cut your fish. Make sure that you choose the right substrate for your fancy goldfish tank!
Substrate can cause many potential risk, threats, and
issues. So, the absolute best substrate for your fancy goldfish tank is, none at all! Believe it or not, many long term fancy goldfish owners do not use substrate or will eventually end up removing it from the tank. This means that eventually any waste in the water will eventually be sucked into the filter, which means less work for you. Tanks that do not use substrate are usually much easier to clean and will remain clearer. It can still be very beautiful!
Sand is a far more natural substrate to goldfish. They will spend hours sifting through it for fun. It also doesn't have any small cracks for food or poop to get stuck in, which is cleaner and safer for the fish. That said, use a small layer of sand because too much can get any number of gross things stuck in it. Fine grain sands can be dangerous to the fish and should be avoided. Sand can also get sucked into your filters and damage or ruin them. So, you may need to take special precautions before adding sand. Sand should always be rinsed before being added to the aquarium, and it should never be added while fish are still in the aquarium.
Unfortunately, regular gravel is not the best choice for fancy goldfish. Food or poop can become trapped in-between the rocks, and you will have to vacuum the gravel every week. Fancy goldfish tanks with gravel tend to be cloudier or have ammonia spikes. Jagged pieces of gravel might snag, rip, or tear the long fins on goldfish. If you still want to use gravel then, use a very fine level. The typical amount of gravel found in most fish tanks is too much for your fancy goldfish. Make sure that the gravel you choose is smooth and rounded, and not sharp or jagged.
A few river rocks scattered throughout the tank can look very nice. However, they should not be layered like a substrate. It could be very difficult to clean, food could become trapped in places that neither you or your fish can reach. Ultimately, it may look nice but, it's just not practical.