Im starting up a 1.25 gal Betta Cube from Petsmart. I was going to use the Red Sea Nano filter that hangs on. It is rated for 3 gal and 15 gph. I think it will work out great but was just wondering if that would be enough? I am thinking about putting sexy shrimp and maybe some zoas and stuff. Use the 50/50 10w coral life bulb.

I used the same bulb on my Bettacube, but I used a Whisper 110 for flow. I don't think you'll be happy with the Red Sea Filter. I don't think the flow would be enough imo. My best advice for you would be to get the biggest Aquaclear HOB filter that fits on the bag of the tank. You can put your heater in there and save space in the main display. Hope this helps.