# when quoting number of gallons, do you quote full tank?

I need to explain as my question is probably not very clear. (brace yourselves for metric explanations as well, sorry)

When we were setting up a new tank, we counted that it would have 72L (72,000 cubic centimeters) because it was 30cm high * 30cm wide * 80cm long. But when I filled it, it was only 50L of water. I didn't fill it to the top (left out about 5cm), and there is about 2 inches of substrate.

So, when you quote your tank size on the forum, do you quote the theoretical capacity, or the actual volume of water?

Luniyn
Generally it seems that most of us go by the dimensions of the tank rather then actual volume of water. I mean when you think about it the plants, caves, rocks, driftwood, heck even the heater or whatever else you have in there means less water is being added to the tank. But could the fish live happily without all that stuff in there? Yes they would survive but they wouldn't feel safe and probably be pretty stressed out and most likely die at an early age. So the amount of water is important but there are other factors that play into a good home for fish. So yeah, when calculating size of fish/gal (or L) of water, go by the size of the tank and not how much actual water is in the tank. And again just like that "rule" is subjective based on the overall size of a fish (i.e. a 20" fish would not be happy in a 20 Gal tank) you have to use some judgment in thinking of a good home for whatever fish. For example, a freshwater dwarf puffer which only grows to 1" in size as an adult, likes to have 3 Gal of tank space. Is that actually 3 Gal of water? No... because they also like a VERY heavily planted tank. So basically they would like to have a 9" x 9" x 9" cube of space alloted to them, but if it was all water they would be very unhappy. The same way your home may be 2000 square feet in size, but do you discount the area that your bed, sofa, tables, shelves, fish tank take up in that size? Same kind of thing applies here.

Yeah, that makes sense (your analogy with the human house-holds). Although the inches of fish per gallon applies to actual water volume I presume? I think the two get confused in discussions and it could be that people are much more overstocked than they think. Was certainly a discovery for me to have dropped 20L from my initial assumption

Luniyn
Not at all, the actual size is what we guage by not water volume. So in your case you calculate for your 70L tank not based on the amount of water you add. Remember that a lot of fish like cover and spend a lot of time hugging and weaving in between the plants and decorations. In a lot of tanks all the open space is wasted because a good deal of fish just don't feel safe there. There are those that like and need the open space. Danios, for example, like to zoom around in the open space at the top of the tank. But even they like to go into the planted area and chill (as much as those hyper things can actually relax). So no, don't worry about actual water in your tank, just go by it's size.

Alright, so the surface is more what counts (provided they have enough height to swim around, of course). Then I can proudly be the owner of a 72L, a 30L and a 60L tank again!

Jimold
I think the only time it's really importand to get an accurate measure of how many gallons is when you're medicating your fish (or adding chemicals). If you figure you have a 70L tank, but there's really only 50L of water, in theory you can over medicate your fish.

COBettaCouple
I just go with the tank size. Aside from the volume displaced by things in the tank, there's evaporation and even the fish themselves displace water. Just don't feel like whipping out the super computer to calculate my true volume (or emptying and refilling the tank from measuring containers).

I think the only time it's really importand to get an accurate measure of how many gallons is when you're medicating your fish (or adding chemicals). If you figure you have a 70L tank, but there's really only 50L of water, in theory you can over medicate your fish.

Good point, that. Very true. Here I was thinking water volume didn't matter at all anymore.

Luniyn
I think the only time it's really importand to get an accurate measure of how many gallons is when you're medicating your fish (or adding chemicals). If you figure you have a 70L tank, but there's really only 50L of water, in theory you can over medicate your fish.

Good point, that. Very true. Here I was thinking water volume didn't matter at all anymore.
That is true for some of the medications, and if the med needs to be exact (i.e. isn't just a tablet that you drop) it will actually say to calculate actual water in the tank minus rocks, decorations, etc. However, unless all of your fish in your tank are sick (you shouldn't medicate a healthy fish, nor expose them to the sickness more then they already have if it can be helped), you shouldn't treat the sick ones in the tank anyway. They should be moved to a bucket or temporary tank. Adding water from the cycled fish tank to the bucket and only an airstone for aeration. Do about 50% water changes daily while treating with whatever meds based on that bucket/tank size (since there won't be any displacement other then the distance from the top of the bucket/tank). But yes, if the meds you are using actually say to calculate for actual water in your tank, and you are treating the entire tank then follow those directions to the letter.

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