Polyp Lab makes a dirt cheap lens. I think I paid $20 for a lens set a few months ago. These are taken with my iPhone. They’re not perfect, but if you intend on trading corals on here, it’s really nice to have!
I believe they sell them as sets. The lens I used for these photos wasn't heavily filtered. They sell one that's similar to using an orange pill bottle. For around $20-30 you can get everything you need.