Stop panicking. This is easy.
1) Get the fish.
2) Put fish in tank.
3) Take care of tank normally.
4) Do frequent Nitrite/Nitrate/Ammonia testing.
5) If the numbers don't look good, increase the water changes until they do.
6) If the water changes are excessive, buy more filtration.
You might want to get more driftwood or something, as I think most polys will appreciate having places they can hide.