Hydro plant choking out other plant

I have a 55 gallon dwc res with 4 mother plants in it, and a while ago I noticed one of the plants looking a little down so I checked it roots, they were bound up with another's. The sad plants roots aren't as strong looking as the other 3 and I plan on replacing it but until then I need to make due. So now i have put the mothers into flowering, which i know means they aren't mothers anymore, the same one is getting droopy again and its roots are bound to the neighbor's roots again. My question is just for the sake of space and whatnot should I trim the roots of the one that is sucking the life out of the droopy plant?

 

Forgot to mention that on the first drop down I did de tangle the roots and everything got better

 

Is it just too much? I mean it is a huge reservoir at 55 gallons. They were growing great then that one just kind of puttered out. What's the max youd reccomend?