Help troubleshooting what's wrong with my American lotuses?

Last year I developed a mini natural swimming (more like soaking) pool/wood fired hot tub design and installed it in my back yard (I know, this isn't a pond, technically, but I figured you all would have the best idea of what's going on with lotus plants). The pool is 6 foot stock tank and there is a two-part natural filter system made up of IBC containers.  a The first filter holds cattails and these seem to be doing fine, while the second filter is where I keep American Lotuses. It's a pretty cool design, and assuming I can get it working better, I'll have to do a write up some time.

The lotuses currently aren't looking great. I started some from seed and others from tubers I purchased. There are some large coin leaves, and the ones from tubers appear to be doing a bit better. However, some of the smaller leaves have started to brown and a couple are even slowly skeletizing, where the leaf looks like its burned away and just the net-like veins remain. The bigger leaves look like they're still ok, but are slowly turning lighter shades of green and beginning to look chlorotic. My hunch is that I'm not lacking in nitrogen or phosphorous, because the algae seem to be doing just fine :/ From what I've read, it seems like these might be micronutrient issues. However, I've read that under no circumstances should I fertilize my lotuses until they have their first aerial leaves. But it's been over a month now  and no aerial leaves in sight, and there are no shoots coming that make it seem like there will be aerial leaves any time soon.

I feel a bit stuck here, and these lotuses doing well is critical to my natural filter design. It's night time now, but I'll make sure to post pictures tomorrow. Thanks for your help!

What are the lotus actually rooting in?  My hunch is that lotus is a relatively large plant in the wild, with the roots running for long distances in bottom mud.  Perhaps if there isn't enough mud in the containers for them to get sufficient nutrients, or perhaps you are trying to grow them in some shallow depth of medium which is deficient in something?  Perhaps you will have better luck with your system by focusing on smaller, "weedier" pioneer aquatic plants.