I haven't had much experience w/hydro. I started out growing some lettuce to get a feel for it and now am trying to grow an outdoor tomato plant using a drip system in a 5 gal bucket.

In my experience with the lettuce I could put about 1 to 2 mL of DOWN into the 1 gal bucket I was using and would see a small change in pH i.e. ~7 down to ~5.5. After a week of having the tomato growing I checked the pH and it was high (7 - 7.5). I added 2mL of DOWN to 3 gal of solution and checked. It was below 4! So I added 1.5mL of UP. Now it was so high something in the solution had precipitated out giving the nutrient solution a cloudy appearance and the pH turned out to be >9 (never saw this in the smaller system even using 2 to 3mL of UP)! So I tried 1mL of DOWN and it was back below 4 again.

It never changed so much with such a small volume of acid or base in the smaller 1 gal bucket. Is it possible that the plant has just used all of the nutrients in this short time and the solution has lost its buffering ability? The plant was already about 3 ft tall when I transplanted it into the hydro setup. I don't have any way of testing the conductivity or TDS.

P.S. can you tell anything about the nutrient concentration by checking the specific gravity with a battery tester or something similar? How about ohm testing it with a multimeter?