I've been reading some postings here and on other forums, and I've found everyone to be very informational!! So, that being said, I figured I'd take my situation to ya'll "experts". I have an approximately 200gal cattle trough that I use as my "pond". Due to old tree roots, my backyard isn't suitable for an inground pond, plus there is entirely too much heat and sun in the middle of summer. So, my trough sits under my bedroom window, which is nice for the soothing sound of the water. Here's my problem(s):

-I have 4 koi babies (well, more like teenagers by now-about 4-5" long) that we've had since last summer. I've been told that they will only grow as large as what contains them-is this true??? I'm afraid they'll grow too big for the trough, and I'll have to splurge big money on a new home for them!!

-I use a submersible pump inside of a housing with 2 layers of filtration media plus the little balls in the bottom. I have 1 water lilly, dwarf papyrus, corkscrew rush and 2 other water plants I can't name. However, every 3 months or so, I have to change out the water due to the sludge. Should I just get a sludge extractor?? Does it stress the fish when I change their water?? Or does it stress them to have yucky water??

I guess, bottom line is should I give up on the trough and suck it up and build a bigger pond with better filtration?? It's tough dealing with filtration issues in the hot summertime, and I don't want my fish to outgrow their home.

All help is greatly appreciated!! Thank you!!