fishtank is definitely still alive just private now.
still filled with wonderful people and I don't regret joining at all but I don't know who you'd have to message for an invite 'cuz considering inaba isn't really around the forums anymore (he's banned) you'd have to ask someone in it to ask for you. it's my main one as it's the longest one I've been in and it holds a nice place in my mind even if I don't really play or genuinely talk with the host or anyone else much at all lately
retro arcade is indeed still alive too just the topic hasn't been bumped at all and it probably went private as well.
also filled with great people. you'd have to talk to steve jenkins about getting into that one.