Bing, me too! It's not only physical space, it's O2 consumed, poop produced, etc. Here's info taken from Americansportfish.com. It sums up what I was going to say better.

"The amount of vegetation they will consume depends upon several environmental factors such as water temperature, water chemistry, and the kinds of plants available. Consumption rates also vary with fish size. For example, until they weigh about 6 pounds, grass carp may eat 100 per cent of their body weight per day in vegetation. Larger grass carp consumes less in relative terms but do consume larger absolute quantities. Fish weighing up to about 13 pounds will eat 75 per cent of their body weight each day and above 13 pounds they slow down to about 25 per cent of their body weight each day."

The way that I reason, I'd rather have a 6# fish eating 6# of weeds and using O2 for 6# of fish flesh than a 24# fish eating 6# of weeds per day and using up enough O2 for 24# of fish. You get the idea.

Going by those numbers above, once they reach 15# start replacing them for the most "bang for the buck".

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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).