When I converted my swimming pool to pond I thought cool, I'll use my DE filter and have crystal clear...it clogged in just a few seconds. After a few cycles of back washing and adding DE with the same results I figured the DE isn't useable in a pond.
Unfortunately that was many years ago before fluidized bed filters (also called moving bed) were popular, so I removed and tossed the filter. Today I would remove the mesh from the filter and add a media like Kaldnes Media, made for fluidized beds. You would need to make a mesh strainer for the out flow inside the tank and I'd put one on the in flow too just in case. It's expensive but it something you can add to. Even though your filter is large (I assume) it's not like you have to fill it with media. And later, if you ever did start measuring ammonia you could add more.
You never even have to back wash. It stays clean indefinitely. The non moving stuff can cog and channel.
The media is a bit expensive but you're getting everything else for free. So for $60 you could have a state of the art bio filter. You might be able to find other positively buoyant plastic things that could work. Army men? Curlers? Small slices of PVC, ABS or other pipe would work great. Not sure they would be any cheaper.
In my case I never did need any bio filter, most water gardens don't need a filter. But I still would have converted the DE to a fluidized bed filters just because it would be so cool.
You have me thinking now for my next pond.