[GpsFrontier]

Hello Gjn,

5 gallons is the total volume

Considering the size the plants are right now, 5 gallons is probably fine. But with 12 plants that will get medium sized in the system, you'll need to increase the water volume to a minimum of 18 gallons (1.5 gallons per plant). I personally try to double the minimums. For the plants you have going, I would probably use a 32 gallon trash can for the nutrient reservoir.

The current level is minimal, maybe 1/2 inch? The system drips water into rock wool cubes that are supported in net pots by grow rocks 24/7

Just to clarify, you built a combination Drip/NFT system. It drips from the top to the plants so it's part drip system, and the 1/2 inch of 24/7 flowing water at the bottom qualifies it as part NFT as well.

The links are David's Grow and David's Base, I dont know of any others, so I'm open to recommendations.

I just asked because I wasn't familiar with them. I wanted to make sure they were actually hydroponic nutrients. As well as see if they had a mixing chart to see if your mixing them correctly. I took a quick look, and they are hydroponic nutrients, though I haven't had time to look for their mixing chart yet.

I had been doing a total change about every 2-3 weeks

Considering the size of the plants and total water volume, that's probably fine up until now. But if you don't increase the water volume, you'll need to start changing them out sooner. You'll need to increase the water volume soon anyway.

I read that a full water change doesn't need to happen.

Commercial operations have ways of getting around doing full nutrient changes. But your system is a recirculating system, and even if you paid for lab testing it wouldn't help because you can't separate your nutrients into separate individual mineral salts. When you add nutrients your adding a balance of all 13 necessary nutrients. But the plants don't use the nutrients in a balanced manner. They use more of some and less of others. When you just add more nutrients your not only adding the nutrients the plants used more of, but your also adding the ones the plants didn't use much of as well.

Think of it like this. Imagine getting a bowl of mixed vegetables to go with your dinner. But you don't like all the vegetables equality. So you pick out the ones you like, and leave the rest. Night after night you get the same bowl of mixed vegetables, except you don't dump out the ones you didn't eat. You just keep adding more of the same mixed vegetables until the bowl is full. Night after night you pick out the ones you don't like and leave the rest. It wouldn't be long before the bowl was full of the ones you didn't eat, and very few of the ones you actually eat. Slowly but surly the balance/ratio of mixed vegetables is thrown off because you never dumped them out, to the point there is nothing in the bowl but what you don't eat/use. In the case of hydroponic nutrients it's the same thing, this balance/ratio is thrown off to the point where the nutrients the plant uses most are depleted, and the ones they don't use reaches toxic levels.

I add about 2 TB of each nutrient every week (that's about 1/2 of a 4 gallon dose)

If I understand you correctly, your nutrient strength is too concentrated. It sounds like you started with a full strength nutrient solution, and have been adding 1/2 strength to the same nutrient solution each week. That means by the third week your nutrient solution is almost 250% strength (2.5 times as strong) because you started with 100% and have added 50% three times (a total of another 150%). I say almost 250% because the plants have depleted some, but nowhere near that much. If your adding 50% strength each week, you could have done a complete nutrient change every 2 weeks withough't using any extra nutrients. Besides that, for plants that size you shouldn't use the nutrients at 100% strength to begin with. For plants that size 50% strength is plenty.

I would anticipate maybe 71 day/ 67 night. Water temp is around 68-70 around the clock

OK, From your earlier statement "The mechanics sit next to a furnace so that the water is fairly warm during the day but most likely gets a bit cooler (68?) during the night." that sounded like the temps were much higher during the day than the 68 at night. 68-70 around the clock is perfect for the water temps, but if that's the air temps as well, that's a bit cool and will result is slow plant growth. If you can get the air temps up to around 80-85 during the day, and keep it 68-70 at night, that will increase plant growth quite a bit. Just don't let the water temps rise, that's perfect at 68-70 (both day and night).

My gut is telling me that my problem is light, but I dont necessarily want to use 600 - 1200 watts 12 hours a day.

You don't need anywhere near 1,200 watts of HID lighting. If you reconfigure your setup a little so all three levels are at an equal height, a single 150 watt halide in the center should be all you need. Put the higher light requirement plants in the middle directly under the light, and the lower light requirement plants around the edges. By raising the lower levels up so their all even, that also gives you space that you can use a 32 gallon trash can for the reservoir.

You could use florescent light as well, but I would recommend at least one 4 foot twin bulb T5 light fixture per row. More if you use T8's. However the electrical cost to use florescent's would be higher than HID. 6 T5 bulbs (52 watts ea) total 312 watts. Twice the wattage of HID. Even if you upped the 150 watt HID bulb to a 200 watt bulb your still using 1/3 less wattage/electricity than florescent.

Here's an electricity cost calculator so you can estimate your monthly electrical bill. You can just estimate using the national average of 10 cents per kilowatt hour. Or if you want to be more exact, you can find the amount you pay right on your electric bill, just make sure to include the surcharges.