I've had this once, and it was as a result of a bacterial infection and dead plant material at the rootstock.
I cleaned the system and let it run (without plants) with a very diluted chlorine solution and a plain run with tapwater.
Put the plants back, and that was the end of it.

Whatever else is causing this, don't just leave it. I suggest you clean it out.

Hi greystoke, thanks for the response

Not sure if I would be able to remove the plants as it's an NFT system and the roots have grown all the way off the 'table' into the reservoir. Also, if it is a bacterial infection I guess it will be lurking in the rootzone anyway.

Can I ask how you diagnosed your systems bacterial problem, i.e. what signs should I be looking for?

How about running some H2O2 in my system, would that help?

Thanks!

A drastic pH drop like this can only be bacterial. What else is there that can cause such a reaction? I suggest you look for dead roots, rubbish, etc. Remove as much as you can, so that the H2O2 acts on the bacteria - not the rubbish. Then try the H2O2 at ±2-3ppm.

Thanks for that, I'll give it a go.

I have some non-horticultural H2O2, but it contains Phenacetin so i'll have to mail order some and wait for that.

Meanwhile do you think it's worth using pH up to bring it back in range or not bother?

Below pH 5 the anearobic metabolism seems to stop, but at that level it will hurt the plants.
I've never tried this on hydroponics, but . . . can you get any crushed sea shells or coral sand from an aquatic shop, or otherwise some agricultural lime? Get about a pound and hang it in a sock in the main tank.
You should still look for dead roots, etc . . and remove it. You will - in the end - not solve this problem unless you do.

Nice hobby isn't it? {{gwi:996599}}

Thanks for the advice :) Yeah it's a cool hobby, I've only grown hydroponic a couple of times and learnt a lot.

I'll look for some coral sand at the weekend, can't get it at the local store.

Seeing as my plants are near the end of their life anyway (all the fruits have pretty much developed) I think it's time for damage limitation.

They're all ripening really fast now anyway, do you think it would be better to run on plain water and keep adjusting the pH up? I'm just wondering if the acidic reservoir will affect the taste of the tomatoes, and since they won't be able to absorb any phosphorous at that pH, why not just use water and concentrate on keeping it neutral?

Don't think plain water will affect the anaerobic metabolism. That hub of activity is concentrated at the dead spots. It does not need the nutrient to survive.

Well, seeing that you are going to clean up soon, I suggest to keep the pH up at times. Not to worry if you can't get coral sand, etc. You could even use some plain wood ash!
Don't forget that your nutrient stock is now infected!

Have fun!

Couple of questions:

If the nutrient pH is low, does that carry through to the plant tissue? I.e. would it make tomatoes acidic and affect the taste?

I'm gonna get some H2O2 today - this site says that normal use is 30-50 ppm, and more for 'cleanup' (0.5ml/litre or 5ml/litre)

http://www.blunt.co.uk/hydroponics-manual/doc/growth-tech/oxyplus/use/index.html

So to clean up a bacterial infestation in a live system would I want something in between, i.e. 2ml/litre?

Thanks!

Sorry steddy_eddie, can't really help. DonÂt know if the pH affects the plants taste.
Also . .
Never done a desinfection with plants in place, and never used H2O2. But if those are the numbers they recommend . . . then I suppose you try it.

Let us know please.

So far so good..

I managed to find a guy on IRC who uses H2O2 regularly, so I quizzed him on the best strength to use.

He said he uses full strength (equiv 5ml/Litre) in his NFT system regularly for normal (oxygenating) use.

So i took your advice and scooped out as much dead root matter as possible (there was loads, way more than i expected - thanks for the tip!), then refilled the tank plus 5ml/Litre of the Oxy-Plus H2O2 solution.

I left it 24 hours to act, drained again (scooping out yet more loose root matter) and refilled with normal nutes, and 5ml/Litre H2O2 again.

pH seems to have stablised - so far so good. I might even start using Oxy Plus regularly as it doesn't seem to have any negative effects; boosts oxygen and keeps bacteria/algae away.

I realise that it kills off 'friendly' bacteria aswell but if you use a non-bacterial additive like Hygrozyme then no foul.. I guess

That's very good information. Thanks a lot! {{gwi:998412}}

I suppose that I've been a little cautious in the past. Think I'll try that H2O2 myself sometime.

Update:

I think I must have had a severe bacterial infection - I've used the full litre bottle of H2O2 and the pH is still not completely stable!

The IRC guy advised me to keep topping up the H2O2 every couple of days as it decomposed, which I did. This did seem to stablise the pH, but I have noticed it has dropped again.
At the peak of the infection it would go from 6 to 4.5 overnight, and when I adjusted back up to 6 it would drop back to 4.5 within 24 hours. With the H2O2 it dropped from approx 6-5 over a week.

So I guess this isn't a foolproof method of curing a severe case of bad bacteria (in a live system), but definitely helps to control. I guess the bacteria will get up the sides of the tank and in other areas where the H2O2 solution can't get, and gradually regain strength as the H2O2 decomposes.

I think I'll still use Oxy Plus in future grows (especially outdoor) as I expect it will make a good preventative measure.

Cool experiment anyway!

Cool experiment anyway! {{gwi:1002854}}

Some of these bacteria form spores which are near impossible to destroy. Intermittent dosing - say a week in between - may be more effective. That gives the spores time to come out again, and then you nail them.
Conversely: a complete clean out and removal of anything that's not alive.