Why do a water change?

I was wondering, why exactly do we change the water routinely, like once @ 2 weeks and then each week thereafter?

Is it because of nutrient build-up?

Is it because of mold, bacteria, algae?


I was thinking about this a lot now because I just did a new startup and could not get to change the inital 6.0 pH'd water (2 new sproutlings) until day 28 - believe it or not. And when I did, the water was fine. It didn't smell, it was clear, nothing floating, no mold on the lid or sides, no algae, etc.. There was some minor calcium on the tub of course from the water line but that was it.

So I was wondering at what point I can go for w/out changing water esp. with regard to keeping bacteria from growing.

FYI, this is a small setup, 2 gallons of water in a 3 gallon dwc bubble tub.

I have a shload of roots right now and they are bright white.

I am going to post itwo explanations, to be sure I explain it. You will need to read both explanations.

After the nutes are added to the tank and the pH is balanced, the plants start eating, consuming the food. Logically, the larger the tank, the more water there is, and the more nutes you need. Logically, the larger the plants, the more they will eat from the tank. Logically, the more plants that are in the tank, the more they will eat. And Logically, the more they eat, the more nutes that you need in the tank. The amount of nutes provided is in relationship to the amount of water, NOT the amount of plants. It is the Ratio or Proprotion in the water that is important. The amount of nutes added to the tank are measured by we commonly call a PPM Meter, PPM meaning parts per million.

The Instructions on the nutrients contain the proper amount of nutes to add to the water, usually determined by "per gallon", and not by "per plant" or "size of plants". In the beginning, we start with 1/4 dose of nutrients, because a full dose is meant for full grown plants. If we use a ppm meter, we seek a ppm of 150 to 250 ppms on average for small young sprouts, it varies from nutrient to nutrient. As the plants grow, we then slowly and gradually increase the nutrients fed to them, and consequently we increase the ppm. By the time the plants are over a foot tall, we will be in the neighborhood of a ppm of 600, then 800 on up to 1200 ppm. Some growers risk even higher ppms, watching the leaves tips for yellowing, to know when to back off with feedings. And as we add more food, the plants grow and eat more, indicated by them also drinking more water. As a rule of thumb, we figure if they drank half of the water, they then must of ate half the food. That is close to being right, but not precisely. If the grow room was exceptionly HOT, they can drink more water without eating more food. And consequently, some days, they eat more, some days they eat less, regardless of the amount of the water missing.

As they eat, they do not eat all of the delivered nutes at the same rate, or same proportion, and that changes the pH and the ppm. It also changes what they call the RNC, or RELATIONSHIP OF NUTRIENT CONCENTRATION. That CHANGE is not easy to calculate or measure, like the ppm or pH is easy to measure.
When the nutrients are first introduced or added, the RNC is perfect, or properly proportioned. In other words, there are specific amounts or precise amounts of each needed nutrient. The exact amount of needed Nitrogen, Phosphorous, and Potash, also commonly called Potassium are present and available for food for the plants.

BUT, as the plants eat, they do not eat all of the nutrients in a perfect proportion or at a perfect rate or equal rate. Some days, they want what grows longer roots, some days, they want what makes more leaves. The plants actually nibble at some nutrients in the mix each day, and gobble down some others. Some of the nutrients are not hardly eaten at all, some are absorbed very quickly. This creates an IMBALANCE of the mix. If you add more water and nutes daily to replace the eaten nutes, (CALLED TOPPING OFF) the IMBALANCE INCREASES. If you only add plain water daily, again, Top It Off, you still get the imbalance. AND the nutes not eaten, or the nutes that are increased daily cause a build up of nutes, and that creates a BAD imbalance of the RNC.
When this happens, the water becomes TOXIC and unusable even tho the ppm meter says there is a proper amount of nutrients. The plants say I AM NOT HAPPY WITH THE BUFFET and like a spoiled child unhappy at the dinner table, they suddenly refuse to eat anything. This is called NUTRITIONAL LOCKOUT. This happens even tho the ppm meter says there are plenty of parts per million in the water. The ppm meter does not indicate if the nutrients are balanced or at a toxic level. It just indicates there is plenty of food on the table.

THIS IS WHY WE MUST WEEKLY DUMP IT ALL AND START ALL OVER.
THIS IS WHY WE MUST DRAIN AND REPLINISH WEEKLY, regardless of whether we TOP OFF with plain water, or TOP OFF with water and a fractional dose of nutes.
Changing the water weekly maintains the proper RNC.

The smaller the reservoir, the more often you need to change the water.

Weekly Drain and Replenish

Topping Off the Tank's Reservoir


Daily Maintenance of the reservoir is mostly keeping the pH at the proper level, and keeping the same level of water in the tank as it is used or evaporates. It is also keeping the nutrient mix or nutrient solution at a proper level, a level that is NOT toxic or too strong.
In the day to day course of maintaining the tank two problems occur. Remember how we discussed ppm, parts per million? Then we discussed RNC, RELATIONSHIP OF NUTRIENT CONCENTRATION. Well, we also need to discuss TDS, Total Disolved Salts. As the water evaporates or is consumed, the concentration of TDS becomes stronger, to the point of being Toxic. The TDS increase as the amount of water decreases. This creates the need to replinish the water, to replace the water. As we replace that missing water, we create an IMBALANCE. Hydroponic grown plants eat what they need, as they need it and they leave some nutes to build up and accumalate into a posionious or toxic level.
This creates the need to daily TOP IT OFF. I have already discussed starting Feedings and Draining and Replinishing in this thread twice. It is simple enough, daily replace what you estimate is gone, consumed, eaten, and absorbed.

But weekly, you need to clear the buffet table and start all over with fresh foods. This gives the plant not only a well balanced buffet table to eat from, but also supplies it with FRESH Food that has not been altered by the plants being picky and choosey.


And, there is the Growth Spurt you will get during the Drain, as the roots get a 5 to 15 mintues air bath. It also gives you the opportunity to look for crimped air hoses, scrub the air stones clean and look for debris and trash in the tank that messes with the pH balance.

Also, as the water ages, undesirable harmful microbes appear and grow in the water. You also take the chance of insect larvae and algae appearing, and weekly draining the tank helps prevent those problems too.

its cuz the plant eats it and needs more... why do you water soil. you can just adjust the res and top it off you don't have to change it but w/e works for you.

It's not just that they eat it and need more, or we could just add more. Plants uptake nutes at different ratios at different times. You can measure ppms to see total nute level but you don't know the ratios with that. Changing the res is like a reset - it puts the nute level for "each" nutrient component back to correct.

Illegal Smile said:

It's not just that they eat it and need more, or we could just add more. Plants uptake nutes at different ratios at different times. You can measure ppms to see total nute level but you don't know the ratios with that. Changing the res is like a reset - it puts the nute level for "each" nutrient component back to correct.

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touche` comrade

Roseman, I get what you are saying. It makes complete and total sense. I did not think about it like the plants consume different portions and different nutrients at different times (which I agree with).Thanks for this great information and explanation/walk-through!

dbo24242 said:

its cuz the plant eats it and needs more... why do you water soil. you can just adjust the res and top it off you don't have to change it but w/e works for you.

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Are you saying that you top your water off for like the 3 months or so of grow?

Would YOU want to eat at a buffet...........or imagine a buffet that once a day, brought out equal parts of the same food, day after day, after day.After a while, there would be tons of cauliflower and cetain OLD uneaten rotten food just piling up. When you "top off" you are adding DIFFERENT nutrients for leaf growth, stem growth, root growth, etc, and the plant might not need those or all of those and they just pile up like the cauliflower would at a buffet.

bumpity bump.

Changing your water every 2 weeks is overrated. I change mine once when i go to flowering and once when i flush. So thats about 6 weeks of just topping off with water and the adding back nutes untill ppm is back to the target spot.

clovergs99 said:

Changing your water every 2 weeks is overrated. I change mine once when i go to flowering and once when i flush. So thats about 6 weeks of just topping off with water and the adding back nutes untill ppm is back to the target spot.

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Knowing what is in your water, not just how many ppms it adds up to, is not over-rated.