**Re: Liza's 20L Nano Tank**
keymaker said:

OK, so I dose 2 ml TPN+ every day with 50% wc every second day. Based on James' formula I add 5.9ppm NO3 and 0.31ppm PO4 with each dose. Cumulated amount is 2x5.9=11.8ppm NO3 and 0.62ppm PO4 in two days.

Please confirm that I must then have 2x11.8=23.6ppm NO3 and 2x0.62=1.24ppm PO4 in the tank through the whole week based on the EI rules. Is this correct?

You are figures are correct, but they are the maximum levels, and occur only on the day without a water change.

Here's a simple way to check it - let's just look at Nitrate.

Open a spreadsheet (in Excel or openoffice). In cell A1 type 0.

In cell A2 type "=A1+5.9" (no quotes). This is the non water change day.

in cell A3 type "=(A2/2)+5.9" (no quotes). This is the water change day, where you remove half of the nitrate, and then add 5.9 again.

Highlight cells A2 and A3 and drag the drag handle at the bottom right of the coloured highlight down for 20 or 30 rows. You'll see that if you drag down far enough (over 27 rows) it will stabilise at 17.7 on wc day, and 23.6 on the other day.

Here are the assumptions we've made to create this simplified model:

~ You are able to measure your dosing accurately

~ There is no other source of Nitrate (no fish food etc)

~ The water used for water changes contains 0 Nitrate

~ There is no uptake of Nitrate (by plants or bacteria or any other means) in the tank - hopefully a false assumption!

If 1 is not true then the model is wrong.

If 2 is wrong (likely!) then reality will be higher than the model's output.

If 3 is not true then reality will again be higher than what the model shows.

If 4 is not true (I hope it is untrue!!) then the real figure will be lower than the model.

So, what I'm saying is, yes, your maths are correct, but really, who can tell? 1,2,3 and 4 will all affect the nitrate levels. If your plants aren't growing then add a bit more, maybe they're taking up more than you expected!

Good luck with this great tank,

Mark