What causes conductivity to rise?

[Gilles]

My tap water has a conductivity of about 200-300, but when i don't do any water changes for a few weeks i have a conductivity of 1200 One week from last change and it is now ~750.

My electrodes are off course retested, there are no stones in my tank and i don't have any drift wood.
I dose KH2PO4 and KNO3, but in very small amounts. K2SO4 is not dosed atm.

Any ideas? / Can it hurt?

 

[Tom Barr]

Fish food, leakage from plants, fish waste, small amounts of shell material in the gravel, build up of other nutrients that are not limiting etc.

Few weeks? If you go say 4 weeks, you can use the EI calculator.........this can estimate how far things can go, still, if you trim and export plant biomass, I do not think there's much issue.
Non CO2 planted tanks illustrate this quite well, even if the Conductivity goes up higher.

Regards,
Tom Barr

 

[Oreo]

I did an interesting experiment many years ago in grade school for one of those science fair things that applies here. I was testing the ability of liquids to conduct electricity. Pure water was a terrible conductor. Add in some table salt though and it became an excellent conductor. Now, that was sodium chloride but many of the ferts we use are salts. Not sure if they'd have the same effect but it's easy enough for you to test in a cup with your probe. Grab some aquarium water, test, add some ferts, test again. See what you see.