Zero Nitrates In An Established Tank!

Hey, i've been testing my water pretty much monthly with the API test kits for nigh on 5 years now and very rarely had anything more than 0 amonia, never more than 0 nitrites and never less than 5ppm nitrates (if im lucky, its usually between 10 and 20).

Did a water change yesterday, no different than i've done before, tested this morning and i got 0.25 ammonia, 0 nitrites and 0 nitrates!

Now putting aside the 0.25 ammonia for a minute, the only reason i can think of that would mean 0 nitrates is if all my bacteria have suddenly stopped working their magic for some reason!
But nothing has changed in my tank or filter for a long time, i havnt even rinsed my filter media out for months (eheim external cannister).

I wanted to rule out the possibility of duff tests so i took some water to my local shop for a test and they confirmed the 0 nitrates although they also said it was 0 ammonia, which is odd as my test tube was a definate light green.

Tank is definately not overstocked and nothing has died without me knowing, also all my fish seem just as active as ever and behaving normally.
PH has always been stable at 7.4 - 8 and no change now.
Kh + gh are 11 and 13 respectively.
I do fortnightly waterchanges with 40% RO water.

Does anyone know of any reason other than no bacteria that would cause a sudden 0 nitrate reading? Or a reason why all my bacteria could just die off? I'm totally stumped.....

Cheers
Dan

Live plants having grown / become more prolific and using up the nitrates?

Re the ammonia, has your water company started using chloramine rather than chlorine? Mine uses chloramine which results in some detectable ammonia/ammonium if I *don't* use Seachem Prime which specifically addresses it.

Try testing the tap water (i) after it comes out of the tap, (ii) after leaving it to stand for 24 hours, & (iii) after adding your dechlorintator/dechloraminator; let us know what the results for ammonia/nitrite/nitrates are for each and we might get closer to establishing a cause.

Ok, results of tests on tapwater:

Ammonia- 0.25
Nitrite- 0
Nitrate- 0 (maybe a bit over)

PH- 7 (this is lower than ive ever had from my tankwater, its never lower than 7.4 even with using RO water... i guess this is a whole different kettle of fish though....)

Results after adding seachem prime:

Ammonia- 0.25 (maybe a bit lower)
Nitrite- 0
Nitrate- 0 (maybe a bit over)

I've also tested the tankwater again and nitrate still 0 but ammonia maybe a little bit less than 0.25, still not 0 though

I've just looked back over my results for the last year though and the last time i had more than 5ppm nitrate was march so i do have very low nitrates generally.

Ok so if the ammonia can be explained by the tap water reading then maybe im worrying for nothing about the nitrate, i only have 10 x small fish and 2 x half grown siamese algae eaters in an 80 litre tank.