I suggest you read Gopal and Goel, 1993. The Botantical Review vol 59.
No 3.
Competition and Allelopathy in Aquatic Plant Communities.

It suggest the same observations I and many other others have noted
for many years. In order to tell if what yu are seeing is in fact
caused by this, you need to make sure that the other factors such as
CO2, light, NO3 etc are not influencing the results.

If you do this, you will not find this effect. I would also reject
that plants have this allelopathic effect on algae as well. Does not
matter what plant species you use. All 300 species do the same thing?
Pretty unlikely to say the least.

Regards,
Tom Barr