Sorry about cats. I lost my last fish yesterday and that makes 30 (good thing they are cheap feeders). So no more fish for me until I know what is going wrong. So many things can happen, shock, water, temps and who knows what else.

Good news for you is your system needs to cycle anyway, I would just get by and feed your beds by hand with some worm tea or light feeding of organic compost tea with no additives just compost tea. That can also be a + because that can help kick off the culture in the beds. Maybe just run the return line into drain or large bucket for time being but not let it run into fish tank that way you can keep fish tank offline until your ready again.

Again just suggestions on my part because advice is like...well you know

I would keep the fish tank offline myself until I knew fish water and fish were stable. It is not warm enough here in my view and maybe in Kansas and the bacteria is very slow to boot up in the beds, sun can help but it sure does not happen very fast when it is only 55 or even 60 F. When it get's into 70's + the life will get better don't you think?

Again just my grain of salt, I am no expert