We will use KH consumption as an example in a 200L aquarium:
You have tested your Alkalinity on day one at 8.5dKH
Do not dose again for four days
Taking a re-test on day four gives a reading of 7.7dKH
Your total loss over a four day period: 8.5 – 7.7 = 0.8dKH
Your first step is to decide at what level you would like to maintain your KH.
For the purpose of this example you are aiming for 8.3dKH
This means that your KH level is 0.5dKH below 8.3dKH and you will need to add your Sodium Bicarbonate/Sodium Carbonate mix to raise the level to 8dKH.
Dosage rate is 2.7ml per 100L = 0.1dKH
Therefore, to raise your KH by 0.5dKH you multiply 2.7ml by 5 to give you 13.5ml, then multiply that by 2 (for your 200L aquarium). Your total dose to increase your KH to 8.3dKH is 27ml.
Now referring back to your loss over four days of 0.8dKH. Divide this by 4 days = 0.2dKH per day loss in alkalinity. Therefore, to replenish your daily loss you would need to dose 2.7ml by 2 giving you 5.4ml, multiply by 2 (for your 200L aquarium). Your total daily dose will be 10.8ml per day.
If you are having difficulty with the calculations, you can use Ultimate Reef Supplement Calculators to get you moving.