Figured it out

[muggle0981]

Ocasioanlly when putting arm into tank-would get a circular rash with a hard bump in it

Finally think i figured it out-noticed i was bumping my hand against hairy mushrooms who had white strings out of then when i was doing water change and noticed it kinda stung

 

[blaxsun]

I'll have to watch my hairy mushrooms!

 

[Garf]

Fluid exchange has a risk. Probably need to wear an arm condom.