A story of a betta with platys.
I recently had my betta with platys, I caution you about trying that combination. I have a story for you.
I have a male betta (Fawkes) that always appeared to be mild mannered. He didn't behave much like any male betta I had ever seen. I never saw him flare up, he didn't build a bubble nest, he just hung around my five gallon tank "pretending to be a goldfish."
One day, after several weeks of consideration. I brought home three platys. I chose platys that weren't too colorful so that Fawkes wouldn't confuse them for another betta. That apparently didn't matter.
As soon as there were other fish in the tank, Fawkes became an entirely diffrent betta. He was flaring up constantly, and began chasing the platies around the tank. It looked like a territory dispute. Like he was yelling "Hey, get out of my yard!" But the chasing didn't stop.
Within a few hours, one of the paties had a large mouth shaped chunk taken out of its tail. I began to get worried. I decided to seperate the betta from the platys for awhile. Let the platys get accustomed to the tank before returning the betta to its home.
I waited about 24 hours before tanking action. I read a chapter on dealing with aggressive fish several times before deciding to return Fawkes to the tank with the platys. Like I expected, it wasn't long before things were back to the way they were.
Overnight that one bite became three. The poor platys had become increasingly skiddish. One of the platy's tailfin looked awful. A second platy was also missing a mouthful of tailfin.
But the platys weren't the only ones taking damage. Fawke's tailfin was ripped down the middle and the upper half was beginning to deteriorate. I watched the commotion very carefully durring the time that they were togther. It appeared that all of the flaring and chasing that Fawkes was doing was causing him to rip up his fins. It appeared that he was cutting and ripping it against things inside of the tank, or that all of the action was causing the dammage.
I never once saw the platys chase or attack the betta.
About 36 hours later I decided to seperate the platys and the betta for good. The chasing was showing no signs of stoping and I couldn't stand to see the dammage continue.
Ever since being seperated things have generally gotten better. The platy's tails are almost healed and the skiddishness has largely gone away. Fawkes now routinely builds a bubble nest, and will flare if someone walks into the room unnanounced. Sadly, his beautiful tail is yet to heal.
*****
I was told by my LFS that platys got along great with bettas
.
They didn't tell me that bettas don't get along with platys. 
*****
About the flow of the filter. Changing the water level in the tank so that the water falls into the tank will largely solve the current problem.
