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Sure it will change. As the water level lowers that will also effect the ppm. Depends on alot of things as how much it changes. I top off my rez every 2-3 days and the girls (2) use about 1 gallon/day between the 2 of them and the meter usually reads 100-200 ppms lower than when the last nutes were added. I add and replenish nutes every few days myself. Just keep it in the range you want after each top off and check the ph too.
I found EC flat, pH flat led to deficiencies after a few weeks. But, I only maintain the tank twice a month. No one pH level allows proper absorption of all nutes so a small swing is good for you. 5.6-6.0.
Which way is best is up to you. I'm a Ronco style grower. I like to set it and forget it. I barely overfeed at pH 6.0. Over two weeks EC climbs 0.1-0.2, pH drops to 5.6 and straight tap water returns me to my original numbers.
These are your babies, you want what's best for them but, these plants are incredibly adaptable and resilient. Don't stress out, this is supposed to be fun. Whatever your nute bottles list as a feeding regimen, cut it in half and see how plants react. While many have success with Lucas' 0-8-16 formula, I've had plants say 0-3-6 is too strong. Rather than use EC and pH to match a chart, use it to speak directly to your plants.