Don't know what TDS is? Fed them some algae wafers, but mostly they had plenty of algae to take care of. That is one reason I got them. No changes, nothing added in couple years. No fertilizers. Guess it could be the fish, but tank is 20 gal and I have 2 SAE's, 3 sarpae tetras and 2 danios that have been in there at least two years. The one SAE I have had over ten years. Really wanted to keep them as I think they are fun to watch and did a great job on algae while they were here.