Can u not run the hose through a window instead?
The only windows near the hose tap are windows leading to "cat free zones", two spare rooms I have that I keep stuff that A. Will hurt my cats if they get it, or B. That I don't want broken/ruined by my cats. If I run the hose through a window, that means opening the door to the "cat free zones" and leaving it open, so the cats would have to go away anyways so they didn't get into the spare room/s.
Can you just plant the crud out of it? I don't think it would take much to have plants use more nitrates than your betta could ever produce. Pothos roots are poisonous to cats, otherwise I would suggest that. Many plants that can grow out of the water could probably suit your purpose, I just don't know what they are. Nitrates are not very toxic however, and I don't personally associate anything under 20 as particularly harmful.
Aquarium plants don't seem to like me.. I can't grow anything except anubias and java fern/moss
I've been trying to find a list of plants that will grow with their roots in the water, so I can find out if there are any that won't hurt my cats, but I'm either looking in all the wrong places or one doesn't exist...
I try not to let the nitrates get to 20ppm in my tanks.. Reason being that I collected a sample from the river across the road, (which is considered so disgusting by locals that no one even swims in it) and for the fun of it, tested it with my API kit and got no readings for anything (a tiny hint of orange in the nitrate tube).. Now, while I realise that there are toxins in that river that I can't test for, everything I could test for came back at 0ish (according to API) and it got me thinking, if that river is filthy and yet everything we (as aquarists) test for is perfect.. Maybe I should step up my game in my tanks if the dirty river is giving me lower readings than my tank. Since then I've tried keeping nitrates as low as I can, and panic if I see red in that tube.

I know its probably silly, but that's me.
Thanks everyone for your replies.