Another case of buy first, research later.
You probably have a common pleco, they will reach at least a foot long in a short amount of time and then over it's lifetime it will only continue to grow.
Your Flowerhorn will also reach about a foot in length in a short amount of time. They both need to be moved into a much larger tank, asap. Waiting until the FH gets to be 3.5 inches long to put him in a larger tank is not ideal. Find at least a 55 gallon tank. A 75 would be better.
The betta will be great for your 15 gallon tank, after the FH and Pleco are moved to a larger tank. I know people keep them in cups with under a liter of water, but they actually like to swim and a betta would love a 15 gallon tank.
All of your tanks will need the appropriate heaters and filters. Bubblers are not required, they are just extras.