Thanks colm and grathod!
Yes, that rock under the line in the pic above was raised a little bit. Perhaps 1 inch. Can't think of anything else to tweak, at least not yet.
Once I got the filter running all was going well until I noticed a pool of water coming from under the filter (a fluval g6) after just a few hours. The filter sits in a baking tray, and all the electrics sit on a shelve above the filter.
So not a major disaster. I pulled out the gasket, re-greased and replaced it up the other way 'round. Seems to have done the trick. It's been running fine for the past 24hrs.
One species of fish is what I'll do. Green neons would be great, but pricey. Bird flying paints a nice image, thanks grathod. I don't have the circulation pump shown in the pic running right now. While clean the g6 pumps out more than enough flow. But it won't last for all that long. It a weird filter. It looks nice, but I can't say I use any of it high tech features.