Just curious as to why everyone says not to add nutes back in? Though I'll agree it shouldn't be done EVERY time you add water - if you see that your plants are eating the nutes, why wouldn't you add them back? Like today... I added water - checked the PPM and it was down from a couple days ago when I did the res change. I added in the nutes I wanted back in there and now I'm back to my target PPM. Smidge higher now because of the pH down... but that's no worry to me.
To each their own - but if my plants eat it... I'm going to add it back and make it available to them.
There is really nothing wrong with adding nutes back to a recirculating reservoir system as long as you realise your are adding all nutrients to the reservoir not just those used nor in the proportions they were used. Many minerals are tied up daily as pH buffers thereby becoming non available to the plants. Topping off with nutrients helps to assure these minerals are still avialable. The only concern really with topping off is that people often do such only to avoid making reservoir nutrient changes. Not changing the nutrients regularly and topping off instead and leave to bad nutrient inbalances causing some nutrients to prevent others from being available. Once just needs to find a medium ground. With recirculating reservoirs I always did top offs of both water and nutrients, rather than just water top offs and changes. I always used frequent changes though rather than large reservoirs.
A too high EC does
not cause a plant's roots to become clogged with excess nutrient salts thereby slowing uptake of other nutrients and a slowing of growth. A too high EC will cause different problems depending on the ratio of the ingrediants. It is not so much to too high EC that causes the problems but the skewed ratios of nutrients. IE too much nitrate means nitrogen burning, too high calcium means decreased nutrient uptake because it lowers water uptake, too high potash causes a high tranpiration rate but also decreases nitrate uptake. This is more of a problem with inbalanced fertilizers from the orginal mix or due to nutrient top offs more than a too high EC problem.
You also need to realise an ppm reading given by a EC or TDS meter is not a true ppm reading. It is only an approximation. A true ppm is based uopon the weight of each salt in mg per each liter of nutrient solution. Those ppm measurements are what the manafacturers state you will receive if you mix according to their directions. If you mix according to their directions and then use an EC meter on the new mixed solution the reading the EC meter gives you will not be the actual ppm, but will be lower. Sometimes much lower.
Personally when I used recirculation reservoirs I used 24/7 automatic water and nutrient top offs and pH balancing. Look up cooling tower controllers on eaby. They will control the pumps/soenoids/pinch valves for pH control and nutrient top offs. Leveling devices for automatic top offs are sold on about every reef aquairium on line store. I used many Hach GLI analyzer/controllers with two peristaltic pumps for pH adjusting solutions, a peristaltic pump with two pump heads for nutrient solutions and a Crouzet water level controller and peristaltic pump for water top offs. They ran many times per day and allowed very little flucuations. Now everything is drain to waste except clone cuttings being rooted and back up mother plants.