Agree with GBBUDD and addy1.
Turn that tub into a mini bog filter. You will not regret it. You will be amazed at the results. I had green water all last year. It never cleared up, no matter what I did.
This year I built a bog and within 6 days my water went from pea soup to crystal clear.
It's like magic or some kind of miracle. That's how good it is!
There's another plus to it. No more rinsing of all those filter pads or sponges. Just sit back and enjoy.
If you have koi in there, your bog size should be about 30% of the surface area of your pond. If you just have goldfish, it can be smaller. So if you find your tub bog isn't efficient enough, just add another one. Or you can build one big one.
My pond is 1600+ gallons