What can I put in a 250 gallon Veggie filter that will not eat the roots off
of every plant. I have Water Hyacinth, Hornswort, Iris, Pickerel Rush,Lotus
and Lilies

If you fill it with fish it will not be nearly as effective as a filter....I
have one to 3 fish in my filter ponds ( depending on filter size) ONLY for
mosquito contol.....Unlike the fish in the fish ponds these are never fed.
When I clean the filter ponds every few years I take out any that have been
born in the ponds and start over again. I put ONLY goldfish in the filter
ponds. Jerri

http://www.fringeweb.com/Ponds/JerrisPond