That's really cool!!!! Any reason for that amount of bottom feeders? I guess I'm asking if they're beneficial to the ecology of the pond. (I honestly have no clue about it, I've never stocked any body of water, nor looked into it).
This is 100% about pond maintenance. They base the amount on how big your pond is, how dirty it is now and how clean you want it to stay. The carp eat the vegetation and the catfish keep the bottom stirred up just enough so the sunlight doesn't reach bottom and grow more.
300 seems like a lot until you see them in a bag. Then it seems completely inadequate once you release them. Our pond is over 1.2 acres and 10ft deep so we could have gone as high as 600 catfish, ($45/100) and 24 carp, ($12/each).
This round cost us $303 but we split with 2 of the other neighbors so it ended up being quite reasonable.
The grass carp were already 6-8" long but one of the guys at the pickup was showing us pics of a 37" he pulled out of his pond a couple weeks ago. They are sterilized and won't reproduce so you don't really want to be pulling too many out.
We may want to reconsider our plan to add Bass as that same guy told us how he added 4 grass carp in the spring and the Bass ate all 4 of them in under 5 minutes. They may be nice to fish once in awhile but once they get too big, they will eat just about anything you try to add.