Which litre figure do I use for chemical dosage

[Saxywigs]

Hiya.
This is my first post and my first aquarium. I have purchased a 180 litre tank but am unsure about the figure I use to calculate the amount of chemicals to dose my tank. Do I just use the figure of 180 to work it out or do I measure the amount of water that goes into the tank in the first place. This won't be 180 litres because of the rocks and decorations in the tank. I don't know how accurate you have to be and don't want to over or under dose and harm my fish. Hope this makes sence, thanks.

 

[Allwissend]

For simplicity I would suggest calculate for the entire 180L aquarium volume. This will of course underestimate the actual concentrations but with yours and many tanks the difference will be minimal and not endanger the fish.
There is no magical precise number that needs to be achieved and strictly maintained, that is why the EI method suggests dosing abundant nutrients for the plants and the range is quite wide. There is nothing wrong of course with estimating the actual water volume of the tank and dosing for that. Here are the major factors:

A 180L aquarium would be around 100*40*45 cm or thereabouts, likely with a 100*40 cm footprint. You can adapt the measurements for your own situation. In that situation
- for a 5 cm thick substrate the loss is about 20L (100*40*5). There is some water in the pores of the substrate, but if ignoring that, there is ~11% less water.
- say 3 cm the gap from the top of the tank, it won't be full to the brim, that is another 12L, so add another 7%.
-also remove water for the hardscape , plants , fish but add the water that goes in the canister, sump or hob.
So let's be generous and say 36L or 20% of the aquarium volume is not water... that would mean you are actually dosing 25 ppm (mg/L) NO3 when you target 20 mg/L NO3, 3.7 mg/L PO4 when you target 3 mg/L or 1.2 mg/L Fe when you target 1 mg/L.

 

[Saxywigs]

Thank you for the detailed reply. I hadn't even thought about the volume of all that lot too! I guess I am starting to over think everything now. I went to collect my first tank last monday to find they actually hadn't arrived due to stock and delivery issues and I have to wait till this coming Thursday to hopefully get one. I am currently like a kid waiting for Christmas day lol!

 

[Allwissend]

I sure can understand that feeling. Fingers crossed. Let me know how it goes.