It's common for Lower leaves to drop for a number of reasons.
Light strength Is a big one. But I wouldn't wory unless you see defects in new growth

With mangroves I was informed the idea is limit the leaves to just a few to promote better nutrient removal by the root system. So I've always pruned older leaves as new ones come in. Perhaps yours are doing this more naturally, getting what they need from the water more than the light.

Plants are not going to process the nutrients without leaves. Removing leaves that are not more than 50% damaged will not help the roots. What you can do is trim roots ( just the tips). This will encourage root growth as "auxins" are redirected to stimulate growth. Plants send the nutrients up to the leaves where gas exchange is done through "stoma". So removing leaves limits this process

Not meaning to suggest removing all leaves just older ones as new ones develop. The other thing with mangrove leaves is they require regular cleaning with fresh water.

+1. On rinsing them off. This is because the stoma get plugged. I pull mine out and give them a Good bath. You don't Need to see the creep for them to be plugged

I have been spraying them down with fresh water occasionally. The leaves are in very good shape. So I guess when they get too tall, I will have to remove them from the sump - bummer

Not at all. What you can do is train the plant.

Find a section of the plant that's not overly woody. Between your finger and thumb roll and squeeze plant. Make it pliable without breaking outer tissue. As you do this the plant becomes bendable. Once you archive a 90 secure it with string or what not. The tips will start growing upward again