Change Res or Add to it?

OK I think I got my reservoir in a funk cause for about 4 weeks I had just added the same ppm solution to the remaining amount left. Should I do this or should I drain the reservoir completely and refill or add. I think I remember seeing that some nutes get used up some remain so if I add and add some nutes may get crazy amounts.. correct?

IMO it is always best to start fresh, then you know that everything is in the right parameters and any issues you have is not related to reusing water.

Ideally change rez every 2 weeks, due to salt buildups, scum and all the other crap it collects. I hate changing the rez so I maybe change it once a month.

I do a fresh res change every two weeks, during that time i top up as and when. Its easy for me as i use RO water so i know exactly how much nutes my plants are taking in and can bring my EC up to the level i want. I wouldnt go longer than two weeks without a complete res change.

i change my res every 10 days. but ive known growers who wouldnt change a res the entire grow...just add water and nutes...i wouldnt recommend that tho

Ok I will change it every two that seems manageable.

i used to change my solution like the bottle of food says every 2 weeks, and start again with fresh food .. when you do this you notice how much quicker you need to buy more food
the correct balance of npk, and micro nutrents will be out of sync is the warning message this message scared me for a while but soon i was brave enough to resist it

here i go stright from veg to flower food aswell, no resevour changes, just top ups with fresh water and food and the occasional ph with the same results as constant changes and constant ph adjustments , sometimes its better to leave it alone if its growing great and see what happens