Yes up until recently i had just been topping up with nutrients but as I've been discovering over a couple of seasons it creates imbalances! So from now on I'll just completely change the reservoir water. I dont know that I'll do it every two weeks. I have pretty big reservoirs so I might go monthly or more, depending on what the plants are doing.
Right now all my chilies are going nuts! So much growth and pods coming through! Wish I had been flushing the reservoirs completely months and months ago.
I tend to find about a month is all i can go until they look like they really need a change.
I don't know how big your reseviors are but I have 2 70ltr tanks connected to my channel with about 6 plants and they will use up all the nutes in 2 weeks max during summer/fruiting stage.
Also I have found during the cooler months and also after winter I will do a water flush out with diluted Hydrogen Peroxide to keep away any bacterial diseases and also mould from forming.
Happy Chilli Growing,
Micca