I'm going to diverge dramatically from everyone else and say that in this tank it is important that you don't have much flow. This is because I can see a betta in this tank. They really struggle with a lot of flow and should be in very minimal flow scenarios. Putting a powerhead in a tank with a betta is basically going to kill it. If you want to put a lot of flow in a tank you're going to need to move the betta. The idea that the bubbles need to blow all around a tank comes from the idea that the bubbles themselves help the plants by attaching to leaves. This is purely anecdotal and there's no real information saying that it affects anything. The dissolved CO2 is what definitely helps the plants, it's possible that more flow will help dissolve even more CO2, but there is an issue of whether that actually matters for your purposes. If all you're trying to do is grow healthy lush plants and not trying to min/max every single attribute, then minimal flow with a diffuser and a small tank like this will work just fine. Alternatively, you can ditch the diffuser, build yourself a small reactor, and have significantly better CO2 dissolution and efficiency then a diffuser could provide.
