Can you take a photo? I have never seen pyramiding on a slider before. Usually that's a tortoise problem...but I do know that what causes it with torts is calcium defeciency caused by either not enough calcium or not enough UVB which they need to metabolize vitamin D / calcium. There is also a good chance that your UV bulb is too far away or insufficient. If it is a screw in type bulb, it is the wrong kind all together. Petstores are marketing a screwin UV bulb that is UVA like plants need and some reptiles need a certain amount of UVA as well but also and mostly need UVB which these bulbs do not produce. Also even with a good bulb, the further away the bulb, the less UVB actually makes it to the critter and it doesn't have to be more than a foot or so away in many cases to render the bulb totally useless. I don't keep water turtles any more but I have. I do keep a sulcata though and I give her UVB all winter but leave her outside all spring, summer and early fall. 15 minutes of natural sunlight = about a week of good UVB bulb
Perhaps pyramiding isn't the exact problem? There are other vitamin defeciencies that cause deformities. It sounds like you are doing great with diet except goldfish are toxic to most animals. They won't kill them right away but they aren't the best choice of feeder fish and there are a lot of other choices equally as inexpensive. Even shiner minnows or shad from a bait store would be a far better choice. The most common problem with sliders in captivity is Vitamin A defeciency but I don't think it causes carapace deformity. The first sign of Vitamin A defeciency is watery and/or swollen eyes, then lethargy, then RI...then a dead turtle. Please take a picture and post it. If it is pyramiding or any other deformity...you can correct and stop it from progressing but rarely if ever can actually reverse the damage.
