Is it bad that my fish are.....?

My pregnant female panda platty is swimming towards the filter. Last time I had this problem I got some stuff and they stopped doing it. I got the same stuff his time (special blend stuff that smells like pure ****) and it is still doing it. I added the correct amount yesterday but then I did my 8 gal water change from my 40 gal. Instead of the water change then the medication. A few of my guppies are doing it to! Im very worries! I don't no the exact fter brand but it has 2 separate places where it shutes water out and I believe cam with a 55 gal. There are 2 air pumps, and I wi admit, it might be a little over stocked or just on the edge of being overstocked. I'm working on getting rid of some of my fish and in a few weeks, i will be buying my friends 75 gal and setting it up As soon as I sell the lizard that he has. Unless he wants the lizard but still I will be buying a 75 gal soon. And putting all my 40 gals fish in. So why are they swimming towards the filter? Last time I got my water checked was about less than a week ago. And it was all perfect except it was cloudy bcuz of a bacteria bloom I think. But it's almost crystal clear now. Do I have to do one more wTer change even tho I just did yesterday? Wi my fish be ok. I do remember them saying that last time the fish did this it's becuz there lungs are swollen and it's hard for them to breath or something. Wat is wrong and wat medication fshould i get for the water??!!!