Strange nitrates

[texa]

Wondering if anyone can shed some light on a problem for me, I'm stumped!

I've had my 150L axolotl tank running for 4+ years now, two adult axies, with a Fluval external 306 filter. Sand substrate (moved about and siphoned every week), fresh plants, and plenty of rocks/logs and toys.

I do a weekly change, around 30%, and every two weeks rinse the filter sponges (in tank water!).

Just recently, I have been getting strange rises in nitrate, higher peaks than normal. Nothing serious, just a rise from ~5mg/L to approx 10-20mg/L, which appears a few days after each weekly clean. Such cyclic rises in nitrate is not something I'm used to seeing in the tank, usually it is very stable and constant even after a weekly clean.

Initially I started doing small water changes every day to bring it back down, which worked, but it doesn't seem to totally fix the problem long term. I've added more filter media to try enhance my bacteria population, and the only thing I have changed in the routine is a swap from Tetra AquaSafe to JBL Biotopol for dechlorinating the water.

I can't think why a change in dechlorinator would cause these rises, but I can't think of anything else!

 

[LSuzuki]

Is there any chance the tap water has changed? Over here, some places use chloramine (ammonia and chlorine bonded together) instead of chlorine in the tap water, and so once your dechlorinator breaks the bond, you have a lot of ammonia in the tank, which will get converted to nitrite then nitrate. Or, if your water now has some nitrite in it, that would also get converted to nitrate.

Is it possible your nitrate test is old and giving bad readings? (I don't know if they would read high if the test is old or not - I'm just guessing.)

Is the wood deteriorating and adding nitrate to the water? (I'm guessing here too.)

 

[texa]

I thought that too, and tested the tap water, but no signs of strange parameters coming from that either.
My test kit is a little old, but I have been using brand new strips as well to make sure there's nothing fishy going on, and the readings are the same.

However, I've had an idea! I have loads of elodea in the tank which I normally remove from their weights (the stems are wrapped in sponge then a weight), but the latest batch I got I haven't done so. It might be the stems are rotting underneath and causing the elevated levels of nitrate, so I'm going to do some gardening, a water change, and test tomorrow to see if that helps!

Thanks for your ideas LSuzuki, I will remove the wood today as well, to cover all bases!