I need to find a manga/anime! It's an almost shonen-ai with a guy who finds a merman and l...

orenchi no furo jijou? cmiiw

you can find it at kiss anime dot com, I know what you're looking for but I forgot the name, you can browse the shounen ai page and you can easily find it thriugh the titles (=・ω・=)

"This boy caught a merman" is the name of that anime.

Here is a manga (yaoi) about a merman (▰˘◡˘▰):
http://www.mangago.me/read-manga/ever_after/mf/c003/

http://www.mangago.me/read-manga/orenchi_no_furo_jijou/