There doesn't seem to be a translation on dict.cc. Is there a German equivalent of this phrase, or would you just have to use another verb or rephrase it in some way?
For example,
You really should do something about that mosquito bite.
Du solltest diesen Gelsenstich handeln.
Would this rephrase be more or less equivalent, or is there a better way to express this?
Your translation is almost correct: Depending on the country, I'd replace "Gelsen", because that is a local term. "Mückenstich" is a more generic term in German-speaking countries. And prefix "handeln" with "be-", making it
"Du solltest den Mückenstich behandeln (lassen)."
You could also say
"Du solltest etwas wegen des Mückenstichs tun/unternehmen."
Then, there is another way to say that, although it is somewhat colloquial:
"Du solltest etwas gegen den Mückenstich tun." (colloquial because of the construction "gegen...tun" - one can hardly so something "against" ("gegen") a mosquito bite; one can do something "about" ("wegen") it. But as "gegen...tun" is often used in spoken German and - as such - perfectly fine, I'd like to mention it here.)
