anacharis will thrive in 76-78F, anything higher will get you mushy leaves and melting. also adding Excel will melt it.
for mine, it grows to be scrawny and not as bushy as when I got them, but they are green and thriving in growth. I think my water is causing the scrawny growth. the shrimps feed on the anacharis too, especially the babies.
try adding some liquid fert with lighting once a week, Aqueon Plant Food is pretty good. Or try liquid Potassium and Micro ferts. I get mine at pfertz.com. The owner has cancer and is clearing out his stock. But you can read up on his site for more info.