[Does anyone know what limpet eggs look like?]

 I found some strange spots in my goldfish tank and I would be thrilled to have more limpets because that tank has never been so clean, but I'm also worried it's something else. Sorry for the cloudy photos, I saw them right after a water change. 

[Help with serious SBD in goldfish.]

I love Dr Erik Johnson! His videos on the 20 steps to diagnosing fish are so helpful and I have his fancy goldfish book. He has a lot of great suggestions and I don't feel like enough people know about him 

[Mystery Snail Stuck?]

Could be the Excel 🧐. And you're sure it was eating enough? My mystery snails eat almost the same as my goldfish! I feed them repashy super gold with calcium mixed in and some Hikari crab pellets almost every day. They are big eaters and I never found algae to be enough for them. I suppose this is the hardest part of the hobby, not being able to tell exactly what happened and it happens so quickly! I did get some lovely mystery snails from a hobbyist on Facebook and they were way cheaper than the store. I hope you do end up getting another one, they are my favorites!

[Help with serious SBD in goldfish.]

There is a YouTuber called life with pets (I think!) who had a similar problem with one of her goldish. She ended up putting it in a shallower, smaller tank and it helped some. There may be more videos on the channel! Aqua Girl Talks also has some videos. Have you ever tried Repashy Super Gold? I was so tempted to buy a goldfish online, but I ended up at my local LFS and bought a derpy ranchu after watching all the fish for an hour. So disappointing they didn't send you the video! If I ever buy online, it with be with Zhao's Fancies. More expensive, but they all have videos 

[Mystery Snail Stuck?]

I've also discovered that mystery snails can be the "canaries in the mine" for something being off in my water quality (particularly nitrite) when they have behavior similar to this. Have you done any testing?