[Zapins]

The plant in the first photo is Ammannia crassicaulis which is a notoriously difficult plant to grow. It also grows quite slowly and seems to take a very long time to recover from nutrient deficiency problems. Here is a PlantFinder entry for a related (and similarly picky) species: http://www.aquaticplantcentral.com/forumapc/plantfinder/details.php?id=118 you can read up on its finicky behavior there.

This particular species often stunts in the way yours does and can do so for many reasons. It is sensitive to GH, and doesn't do very well at GH over about 6-8. If you recently added calcium to the tank then that is likely why the leaves are twisted up like that.

It can also develop twisted leaves when nitrogen deficient, though you should see some of the older leaves deteriorate as well if it is nitrogen deficiency. How do your old leaves look?

What ferts are you adding, what is the GH, and any other test kit values you have available (not pH).

Can you take a few more photos of new and old growth?

When you have corrected the problem the plant will take at least 2-4 weeks to start recovering.

 

[Zapins]

Interesting observation with the GH and the buffer. I don't know specifics on the buffer you are using and what is going on chemically but if it lowers the GH and you stopped using it around the time the plant curled up, and all other values and plants are not showing these signs then I think that is likely the problem.

5-10 ppm phosphates are very high. I'm not sure they'd cause curling issues, but it might be a good idea to try lower them a smidge

RO water might be a nice alternative to chemicals in the future.

I'll be interested in hearing an update on this plant if / when you start adding the buffer back and the GH goes down.

 

[Zapins]

I find it ironic that the prevailing theory on GSA is that it is caused by low PO4 levels, yet you have 5-10 ppm and GSA. I suppose it goes to show that algae is a lot more complicated than we know about.

Phosphates are all the same, they are all PO4. However, PO4 groups can be attached to many different chemicals and so don't always act the same in chemical reactions. If the test kit you are using measures free (ionic) PO4 groups then plants should definitely be able to use them. I am sure there are several different phosphate containing chemicals plants can use, so it is possible they may be using your buffer's phosphates. Plants just don't use very much phosphate compared to nitrogen or potassium.

Here are some examples of phosphate groups attached to various things so you can see what I mean: