Hi Col, I've been following your thread in the scientific section of 'another forum'
I'm essjay on there. To be totally honest with you, I haven't read it recently with all the problems with google, I've almost given up trying to get on that site.
My understanding is that a nitrate of zero is unusual in a freshwater tank, though it is something
marine tank keepers strive for - though not being into
marine tanks, I'm not 100% sure that's correct.
In a freshwater tank with a mature filter, nitrate is constantly being produced. Plants will remove it, but you have to have a lot of plants to remove all of it. I think what the comment about zero nitrate being rare and something is about to crash means that if the nitrate is zero, it must not be being made therefore something is wrong nearer the beginning of the nitrogen cycle - maybe all the ammonia or nitrite eaters have died.
I did wonder what "exceptionally clean" means - even if every scrap of waste is removed, nitrate would still be produced so they can't have meant that. I suppose if you did very regular, very large water changes with zero nitrate water it would be possible to keep the tank at zero, but that would be a lot of hard work.
I see from the rival site that you have zero tap nitrate and are heavily planted, so in theory for you a tank nitrate of zero is possible. I'm not very good with plants (on the old Thinkfish forum I was famous for being able to kill duckweed
) so I'm not sure what the implications of zero nitrate are for plants.
If you are worried about the implications of your tank nitrate ("something is about to crash") then checking your ammonia and nitrite on a regular, ie daily, basis should reassure you that if something is going wrong you'll catch it straight away. As for the plants needing nitrate, I'll have to leave that for someone more green fingered to answer.
One thing I will say - my son worked for several years for a water testing company and he is highly amused at us trying to accurately measure nitrate with a home testing kit. The company he worked for used very expensive lab equipment to measure nitrate. He reckoned our kits are incapable of giving a result anywhere near accurate.