I understand that the tank does not have any substrate for easier clean up, what you have to understand is that goldfish don't have stomachs like other fish do, so what goes in quickly goes out, this is why goldfish are considered dirty fish that most put in ponds and call it a day. Like @Henders asked about your levels, they wouldn't have anything to do with your fish being scared BUT you should keep in mind that with no beneficial bacteria to grow in your tank, your levels can quickly change due to the fact of having goldfish.
Your question has to do with the scared factor of your fish which I will answer along with some other tips that can help you on your journey.
Fish have confidence levels just like anybody else, if you give your fish what they need, they will feel no need to hide. In a normal setting whether it be a store, pond, lake or whatever, there is substrate. You need the substrate because it holds on to their waste which creates what is called the Nitrogen Cycle, in a short version, you feed your fish, the waste created by fish (poop) is then turned into Ammonia, that ammonia is then turned into Nitrites and then into Nitrate which is then removed when you do a water change. Without any substrate, I'm assuming you do weekly or daily water changes on your tank which blocks the cycle from happening. Ammonia is the #1 killer of fish, if it can't change in to the lesser toxic version of it which is Nitrite then Nitrate, your tank is just a bowl of toxins that shouldn't just be removed by water changes. If you add substrate, your tank will go through the cycles as it should keeping your levels healthy and your tank clean.
By the way if you're concerned about your tank being cloudy, its because of the cycle. If the nitrogen cycle does not happen, your tank will become cloudy going through a daily or weekly syndrome called New Tank Syndrome, it happens because ammonia, nitrite, and nitrate levels are high. The only way to fix it is to leave the tank alone to fix it self and if it's really bad do only 10% water changes.
You should only be changing your water every month doing 25% changes or weekly doing 10%.
Besides the substrate helping with that, you need anchored plants and a lot of decorations to boost your fish's confidence and not hide anymore. Just because you have plants floating does not mean that it's enough.