Six gallons!!! Can I make some playful jokes at you??
Does the six gallon volume include the volume of your pipes and tubes, (i.e. did you start out with 2 gallons)?
Are you sure two clowns can fit in 6 gallons? Oh, yeah... you said you've got a very small hole at the top, so you'll be okay there.
Why not just use your 6 gallon tank as a quarantine tank for your 65? One neon goby at a time.
What do you use for topoff, A shot glass?
Are you going to add two part by the drop? Oh, no, you can just do a 100% daily water change. It'll cost about 50 cents.
In addition to the clowns, I'd think you could add enough invertebrates to double the bio load. Like maybe half a dozen rotifers.
OK, this is getting a little silly and not so funny. I just have a hard time imagining what it takes to maintain a 6 gallon tank.
Good luck, I'm sure it will turn out to be beautiful. All this was meant tongue in cheek, please don't take it wrong.