First of all fish at any level are fish
except there are less fish the higher you go which kinda sucks when you want to take a shot at 400NL and 600NL and all you see at the tables are 6 winning regs exchanging blinds *sigh*.
The only thing that changes as you move up are best regs are generally better the higher the limits you move up to. I would honestly say that 50NL and 100NL plays about the same, even 200NL does, though there are a few tougher regs there.
The whole "I'm getting 3bet a lot more and squeezed a lot more" thinking is almost entirely all in your head so be careful. You probably just notice it more often because the betsizes are bigger and maybe you're playing fewer tables and are more focused in general. I'd say just take notes or use stats (but careful with sample sizes!) to determine who's really 3betting a ton of junk and squeezing light a lot and who just seems to be doing it but shows up with a hand way more often than not. I think one of the biggest mistakes people make at least at 200NL where I put in most of my hands currently is overadjusting. People assume that you're 3betting light or that you're 4bet bluffing way more often than you actually are. I've been guilty of this myself and still do it on occasion and end up regretting 5bet shoving that K2s
The reality is that there are very few people who actually play so aggro that they make your life hell and those guys almost always end up being huge spewtards anyway, at least up to 200NL so I wouldn't even worry about it.