Seroma in a dog

^^ what mike said is what ive always done, a small needle sized hole will close up in no time. I generally cut 1- 1 1/2" wide to allow it to stay open longer to drain. One thing to try to look for is any foreign matter in it that could be causing it to fester. I have one that I just couldn't get rid of a couple years back.... this was after two seperate rounds of meds and 2 seperate cuts to drain it. Finally we decided to go at it our own way and cut straight into it, I ended up pulling out a 3/4" thorn that was laying sideways inside the skin. Soon as we got that out it healed up in no time.