Waste=salts build up in the solution as nutes are used in addition to bacteria and various pathogens as greenthing stated. In a few simple terms...Just as important as a sterile surgery for us, a hydro plant needs the same because her guts are hanging out. You get what you give and it's very important to give in this respect because in a sense hydro girls live all of their life in an open heart surgery. Would you want irrigated with saline or "duck pond" water? Now....That's not to say it won't work, I know a guy who never changed his solution and basicly did EVERTHING wrong including never flushing and keeping 1400 ppm til harvest day and still got some really good yields. He stuck with what worked for him not even knowing the diff. He refused to educate himself on the basics and follow them and was fine with it....until I showed him what the propper process yielded, looked like, smelled like, burned like, tasted like and potency. My run was totally diff in every aspect. Hydro is ALOT of work but shortcuts show up clearest in the bud, besides, It's more work dealing with issues that arise as a result of short cuts than it is to just follow a widely recomended process. Try adding top-offs until they equal the solution volume and see what the time span is. Change solution when you do. Follow that and you will not regret the results.