Have you ever added any fins other than the tail fin to the tails you've made? I've always wondered if it was possible to do that without having to do a silicon tail

Like dorsal fins, pelvic fins, etc? I’m actually working on doing that for the next tail I’m making! I was going to do it for my goldfish tail, but ran out of time and then moved away from my sewing machine. It’s also harder to do with fabric tails since it means accounting for extra seams, unless you’re only putting them into the sideseams. Putting a dorsal up the back is pretty straightforward; adding hip, pelvic, or ankle fins that insert horizontally or at an angle is harder to get right without causing the rest of the tail to wrinkle.

Lots of people (including Hanna Mermaid/Hannah Frasier) actually use vinyl translucent floor matting to make dorsal fins/edge their flukes, since it’s nicely textured, plastic/waterproof, and pretty neutrally buoyant: 

often people trim it along the lines like above to make fringe. The downside is it’s very hard to color it, since it’s plastic, so things like paint will just flake off, and it can yellow over time. If you’re not doing straight fins (like ones that curve) the lines can also make it look odd. You also have to have a machine that can handle sewing it in since it can get pretty thick.

The other option mostly is to use the same fabric as the tail is made from, although this is trickier with because it’s too flimsy on its own imo. So you have to use the fabric as a covering for something else neutrally buoyant. In my monofin extention and the original set of hip fins I was gonna have on my orange tail, I used duct tape, folded across itself–stays pretty dang waterproof, is flowy, and has a nice thickness to it. I know another mermaid who used those plastic school folders, and people have talked about using shower curtain material too. The folders/shower curtains, if you get them colored, can also be used without fabric over them even. Those are personally a bit too flimsy for my tastes; I like something that holds its own shape a bit better. 

I haven’t started them for the tail I’m currently working on, and I’m still debating over what kind of material I want to use–I like the translucent effect and textyre of the matting, but it’s harder to find these days, and I like to have more freedom in the shapes I want. I’m planning on testing out a combination of fabric+duct tape+plastic, but I’m still in Scale Construction Hell (oh god why did I think handmaking individual scales was gonna be fun), so haven’t gotten to that point yet. But I hope to have at least hip fins + one or two more sets of front ones. I personally  am not huge into dorsal fins, since I think they draw attention to the fact that we have knees and fold up when doing a dolphin kick, and you have to be careful when you’re sitting down etc. Since I do so many photoshoots sitting on rock or hard surfaces, I think it’d be harder on my tails and get in the way.

I hope that answers your question! :)