I am setting up a 180 gallon low tec planted tank. As far as dosing goes i would like to know how do i dose K if there are already nitrates in the water from fish/bacterial load? I only intend doing 10% water changes monthly so Tom Barr's EI index goes put the window. I aim for 10ppm K.
Also what is the dose for direct dosing into the water. A stock solution is not feasible for 150 gallons water.

Very informative site.
LAKA

Well, it's going to tough to make any predictive estimation with K+ with such low frequency water changes.

You can buy a relatively inaccurate turbitric Lamotte or Hach Test kit, but it'll give you about a + or - range of 10ppm in that 10-20ppm range

So it's not much use.

Another method is to dose based on the N uptake, but that assumes you know what you add there fairly well also. Fish waste often gets consumed as fast it's produced in aquariums. Those feces drop intop the substrate and very slowly release the NH4/NO3. Generally at ranges that exceed the test kits you might have.
Same with the PO4 from fish waste..
you can try and weigh and measure the fish food before you add it, get an idea of the N/P/K content.

Given your constraints, there's not much you are going to be able to do here.
Just eye ball as best you can.

Note, when you start adding more K+, the NO3/PO4 and CO2 demand will also go up as well.

That can lead to other issues more serious than low K+.

Just warning you.

I'd add 1/4 teaspoon K2SO4, 2x a week, buy a Lamotte test kit, make sure the K+ does not get much over 40-50ppm.

Realize that other nutrients will be uptaken faster now that this is not as limiting.

Regards,
Tom Barr

Thanks for the advice Tom.
According to the "Fertilator" i will need 2.5 teaspoons in a water column of 150 gallons to get potassium redings of 10 ppm.
What i intend doing is adding this amount initially once a month and see how things pan out. If there is no sign of K deficiency on the leaves i will keep status quo. Otherwise i may add fortnightly. It will have to be a wait and see approach.
I have just bought a bag of KCl so i'm stuck with this for the time being.
If i have no readings of nitrates then i will supplement K as KNO3 instead and do without KCl
LAKA

When you say this will be a low tech aquarium, what do you mean? Low light? No CO2? Both? You are planning to do water changes and dosing of some fertilizers, plus a lot of testing, so what do you gain by going "low tech"?

Low tec for me means 2 WPG lights and no exogenous CO2 source. I still feel that as my heavily planted tank matures i will require nutrient supplementation beyond what i obtain from fish food waste water and soil. The fish population will be light.
LAKA

Are you planning to follow the "no CO2" method Tom describes here on one of the forums? If so, you shouldn't plan on doing any routine water changes, or at least limiting them to twice a year or so.

That's correct. Diana Walstad replaces 40% water every 6 months or so and tops up for any evaporative loss. I decided to do 10% change monthly plus any top ups as required.

When you change water, unless the new water has been sitting in the open for several hours, you are introducing CO2 to the tank. This raises the level of CO2 in the tank for a short time, and that fluctuation in CO2 can trigger algae to start growing. Ten percent may be too small to do that, but why take the chance? Ten percent is also too small to do much in the way of resetting the water parameters, such as hardness, that topping off the tank for evaporation has changed. If you are still in the planning stages, and have access to a water line plus a place to drain water easily, you could set up a continuous water change system, that adds a dripping flow of water, with an overflow getting rid of the excess. That seems to work fine in my tank.

Don't bother with the 10% at all then, follow the suggestions on the article, that will address the needs of the plants and prevent other issues from other nutrients besides K+.

You do not need to do any changes is more the point.

Just after a large trimminmg or if you plan on disturbing the substrate/uprooting etc.

Otherwise, hands off.

Do not mix the methods and do 1/2 way with the water changes, you are not preventing any significant build up etc with 10% monthly.

So do not even bother, you don't need to anyway.
You'll dose once a week if you follow the guideline I suggested in the article.
That's all, and the KNO3 sounds like it will be needed, not KCL. You have no NO3 as it is.

So adding a bit 1x a week is fine in a small amount, perhaps 2-4ppm for NO3.
There's plenty of K+ if you do.


You do not need 10ppm etc K+ for such a slow growing tank, the same K+ for such a tank's NO3 uptake is about .5-1.0ppm per week, not much.

Regards,
Tom Barr




Regards,
Tom Barr

Tom
I like the idea of low maintenance. I am more than happy to do away with water changes if averything is in homeostasis. Also if dosing wikth Kno3 will provide all the K i need oaa the merrier.

VaughnH
I never considered the CO2 in tap water during water changes and potential for algal growth. What puzzles me is the daily diurnal fluctuations in CO2 levels related to the normal cycle of photosynthesis. This doesn't appear to be a problem with algae. How much CO2 can 10% water change introduce into the system anyway? I suppose i could aerate the water to be on the safe side but i do not think it will be a problem.
LAKA