Try switching to an "organically-derived" nutrient solution or maybe change growing methods. If you were to use the "static" method, you could top water with only the amount the plants transpire so as not to create a run-to-waste situation and maintain both the proper EC and pH by adding only fresh nutrient.

I know you already knew this; I'm posting my suggestion for the benefit of those who may not have as much experience as each of us.

My PH will go up until the plants mature and then will go down. It does not matter what type of plant, or what method I use.

While the plants are growing, they are developing roots and leaves and therefore use those nutrients. Adding back water usually helps a few points on PH, but not much. To compensate I add PH down, which increases my EC.

As the plant gets bigger, I see a week or two with no change in PH, just a dropping of EC as I am not adding PH up or down.

Then as my plants near the fruiting stage, the PH starts to go down. So I start adding PH up, but since my plant is more mature, it is using more nutrients that I am adding with the PH up.

After a few more weeks, the PH will drop rapidly each day, especially on heavy feeders like tomatoes. Then as the plant reaches a full fruit ripening stage, the PH still drops over time, but much less.

Before the fruit fully ripens, I have seen the PH stay stable for a week or so also.

Because of all of this, I do some things that maybe I should not do, but work for me. When plants are newly transplanted into hydro, I know that the media will increase the PH slowly over time for at least one week. This happens no matter how well I rinse the media.

So when I newly transplant a plant, I set the PH at the lowest level the plant can tolerate as I know the PH is going to go up. I then check the PH twice a day for two days, and keep the PH at the lowest level (5.8).

I then check the solution every two days for PH, and bring it down to the middle range (6.0). While the plants are young and growing I will check the PH daily, but many times will not bring it down until it hits the high spot on PH for that plant or until I see changes in plant color (yellow leaves etc).

I only have to do this for about the first week or so, then it is much easier.

A long time ago I bought a nutrient solution that would have my tomato plants go from a PH of 6.2 down to 5.5 in just 12 hours. I then switched to another brand that was more expensive, and this helped a great deal. None the less, I still have PH swings in my solution.

I also noticed that the more expensive solution was actually cheaper in the long run, as it required 1/3 of the amount of solution to get the same EC level.

One last thing to note is that my tap water has a high level of chlorine in it. If I take my tap water and put it into a bucket and test the PH, its about 7.4. If I let that water sit overnight with aeration, it will drop to 7.2 or sometimes 7.0.

So for me, adding water raises the PH but artificially. I check the PH after topping off the plants. What I do now is keep two 5 gallon buckets of water being aerated that is 24 hours old to actually water the plants with.

I understand freemangreens' suggestion which I'm considering, but I'm impressed by mrpepper's observations. They make a lot of sense to me.
Nitrates are used predominantly during the growing stage, with the result that the pH increases. Then, when the plant is mature, the need for nitrates drops, and the plant starts to use Ca, Mg and K to produce flowers and fruit, and the pH will drop.

I think these observations can be used to determine what to add to the nutrients to compensate for the losses, ie: nitric acid when the pH goes up, and "kalkwasser"(=CaOH) and potash(=KOH) when the pH drops.

Agreed??

Forgot to mention the need for Calcium in the growing stages.

Calcium is a component of cell walls and is also important for cell division and elongation, permeability of cell membranes, and nitrogen metabolism. It is different from most plant nutrients in that it is only moved within the plant by the water moving from the roots through the leaves.
The Calcium concentration of plant tissue, unlike that of nitrogen, phosphorus and potassium, increases as plants mature.

So, the best way to increase pH in the mature stage is potassium hydroxide (KOH).

You can stabilize the pH of your nutrient solution by preparing a suitable buffer to prevent big pH changes. I recently wrote an article in my blog about this, you can find it here (http://allhydroponics.blogspot.com/2009/02/my-ph-balancing-system-for-hydroponic.html). I have grown lettuce and tomato with my solution needing no pH adjustments for almost 3 months.

Here is a link that might be useful: Everything Hydroponics

That's very interesting. On the other hand, I find the pH fluctuations useful to determin the maturity of the plants. With buffered stock you may just lose that advantage.
I like mrpepper's method of starting the nutrient off at the lowest tolerable pH and allowing it to rise to the maximum before considering any adjustment.

I like Danielfp's suggestion on using salt meters to determine the levels of the main nutrients, and then trying to keep them in balance.

I also like the idea of the buffering, but also like Greystroke mentioned, it is helpful to know when the plant is maturing at which stage.

Last year as an experiment I changed my Grow to Bloom when my Tomato plants PH started going down several days in a row. The plants really flowered a lot, and produced tons of fruit.

This year is my first real attempt at growing both Brandywine and Beefsteak Tomatoes in hydroponics. I am going to try the switch from grow to bloom based upon this PH change and see what happens as I hear both plants do not normally produce a lot of fruit in hydro.

I am then keeping some seeds, and see if I can improve the growing ability in hydro by allowing these to naturally adapt over several life cycles in only hydro.

I thought I may add that since I am doing DW, I have to remove my plants from their buckets in order to change the PH. To do this, I have a spare bucket. My lid, basket, media, air line, and air stone are all a part of the lid. So I just lift the lid, and with it I get the basket, media, plant of course, and aeration at the same time. I then drop another air stone into the solution, add my PH Meter probe, and then add the PH up or down to it. After it stabilizes for a bit, then I remove the air stone and meter, and put the lid back on it. This way my plant roots are not exposed to the PH up or down solution at strength.

I am sure you guys do the same thing, but just wanted to add that. BTW, thank you for all your comments as this is only my second year (third season did two in a row last year) of hydro so I am still learning.

just remember the seeds you collect will be a cross between the two varieties you're growing. to prevent cross breeding, you're supposed to keep all other variants 1/4 mile or so away, i believe.
I found it extremely annoying to have to lift the lids on my plants each time I wanted to adjust or add anything to my reservoir. That's why I added drain valves to the base of my buckets, that way I can drain a little off to test. I add stuff in at the top, let it circulate a while, then drain off a little more to test again. I found that much more compatible with large plants like tomatoes. once they're 6 feet or so tall, it can be a chore to lift the lids and lug around extra buckets full of nutrients and what not.

I have small 1" holes drilled into each lid. I then have a grommet to plug the hole. I stick my PH probe into the hole to check the PH. If the PH needs adjustment, I just lift the lid, and move it onto another bucket.

Yes on my 5 - 6 foot tall tomato plants last year it was a chore to adjust the PH. But with 5 gallon buckets I was only doing it about once every 3-5 days which was not so bad.

This year I bought some drain valves for my buckets and had thought of using a pump to drain the bucket into another bucket where I would adjust the PH, then pump it back in.

I had issues with my plants when I added the PH up or down to the water with them in it. So that is why I created a system that has it all connected to the lid.

Last year I just put air stones in the bottom of the buckets. But then I had air stones that moved to one side or another, and the larger plants wanted the air directly under their roots. Also the air stones made plenty of noise vibrating on the bottom of the buckets with all the air I was pushing through them.

This year I used zip ties to create loops to the bottom of the net pots. On each loop I put a rubber band to hold the air stone. This way the air stone is directly beneath the net pot, and just off the bottom of the bucket.

My grow room is much quieter now. The pay off is that I make a small mess when I change out lids from bucket to bucket and having to move the air stone around.

I only grow one kind of tomato at the same time when I do seeds. Although I will have other plants in my grow room. I have yet to have a plants cross pollinate across species like squash to pepper.

Not sure if that can happen.

I actually think that these salt probes would be wonderfull, if. . . they could be a bit cheaper. Imagine testing all major ingredients live, and adjust them wherever required. I agree that it's got to be hydroponic's future.
In Africa - however - i'v got a major problen getting my "pupils" to understand a pH-meter. Difficult to explain.
I think I will have little chance explaining these salt probes.

Well, I have found that the best way to explain ion selective electrodes is to make your students build one themselves. There are several articles in the journal of chemical education for the simple making of nitrate ion selective electrodes. I did one with a pencil, just by depositing a layer of doped Polypyrrole over it ! Nothing that can't be easily done at a regular chemistry lab. I use that in my hydroponic projects now,

Here is a link that might be useful: Everything Hydroponics

I've got a chart that I have stuck up next to my plants. It's very simple:

EC Up, pH Down = reduce nutes
EC Down, pH Up = increase nutes
EC same, pH Up = Good

Since you've got both going up I'd say you're very close between the first and last ones. Basically you've got them dialed in almost perfect, maybe just a tiny bit hot but nothing even close to harmful.

Considering that plants tend to eat more as they get bigger, that same level of nutrient would probably be perfect after a little while.

There is a difficulty understanding the swing in the pH.
The way I understand it is that plants eat ions. So, if the pH goes up. The plant has been eating nitrates and/or phosphates. If the pH goes out-of range, the nutrient needs to be adjusted with nitric - or phosporic acid.
If the pH goes down, then the plant has been munching K, Ca and/or Mg. Again, if the pH goes out-of range, the nutrient needs to be adjusted with K2CO3 or some powdered dolomite.
If the pH stays more or less constant, then the plant has been eating from both sides (or nothing!) Adjustment then depends how the EC is moving. If up . . . add some water. If down . . . add some nutrient.

I use aquarium test sets, mostly nitrates and "hardness" (GH) measured as CaO (which includes Magnesium).

The entire process is very complicated and no simple explanation can truly account for everything.

pH even varies with temperature and aeration.

A well buffered solution helps to prevent the normal fluctuations from extending past the target range.