Are Our Tanks as Big as We Think?

Discussion in 'Freshwater Aquarium Builds' started by fish gazer, Jul 6, 2014.

  1. fish gazer

    fish gazerValued MemberMember

    We buy our tanks. Let's say a 29G tank from the store for example. You do your measurements and ya, it will hold 29G of water. Great.

    Then we start thinking about adding plants, decoration, filters, pumps, fish etc, etc.

    Is the tank still 29G? When you consider displacement with all these added components, we no longer have that 29G empty tank that we first purchased.

    So when you think about stocking, medicating, and all that good stuff. I think it's important to keep in mind that we are actually dealing with a smaller tank then we think we have.
  2. _Fried_Bettas_

    _Fried_Bettas_Well Known MemberMember

    Actually it is much worse than you think. Based on the internal measurements, the standard 29 gallon tank holds 25.1 gallons of water. Almost all sizes are less by a similar margin, mostly I think because they use the outside measurements in calculating volume. A 20 gallon long actually only holds 16.5 gallons.

    When you take into account the factors you are referring to. If you add 2 inches of substrate (not uncommon to use even more) you take away another 3 gallons. Your 29 gallon tank becomes 22 gallons, and a 20 gallon long is down to 13.5 gallons.

    It's not hard to picture another half gallon being taken up by the other items you mention.

    So yes, you are very correct.
  3. sbdzxValued MemberMember

    Wow, that's a great point. I never thought of that. :;bmu

  4. Watermark

    WatermarkValued MemberMember

    which is an EXCELLENT reason to not subscribe to the 1" fish/gallon (debunked) theory ;)

  5. Lucy

    LucyModeratorModerator Member

    Very good point.
    One that I have not seen discussed for a while now.

    Thanks for bringing it.
  6. OP
    fish gazer

    fish gazerValued MemberMember

    Forgot about substrate! I think my 29G is actually a 15G in reality, poor fishies...

    Ya, I despise that rule! Doesn't make any sense...
    Last edited by a moderator: Jul 6, 2014
  7. ryanr

    ryanrModeratorModerator Member

    Whilst the volume of water is important in diluting toxins (nitrates for example), the physical dimensions are more important IMO. That is, the swim space provided to our finned friends.

    Take column style aquariums for example, a 30G column, whilst it holds the same as a regular 30G tank, active fish are likely to be happier in the regular tank. Most fish are lateral swimmers, not vertical, so having room to swim and turn around is probably more important than the actual volume of water.
  8. FiscCyning

    FiscCyningWell Known MemberMember

    So now I'm curious... When we dose medications, are we actually dosing the correct amount? I assume the company factors this into their dosing instructions, but it would obviously vary depending on tank size.
  9. _Fried_Bettas_

    _Fried_Bettas_Well Known MemberMember

    This is a valid question, but I would hope there is more than a 20% margin of error in the dosage of medicines.

    I was looking at a chart of the actual volume of various tanks. A 75 gallon tank holds only 65.3 gallons, that is really deceptive.
  10. OP
    fish gazer

    fish gazerValued MemberMember

    This was actually my biggest concern when writing the post. I was always worried that I was overdosing.

    I contacted Fluval, API and SEACHEM just to have an idea of what they have to say about their dosing instructions. I'll let you know when I receive a response!

    Sent from my cell phone.
  11. sbdzxValued MemberMember

    Also, in addition to meds., what about filtration? Like, if you have a canister, you want your tank volume turned over 5-8 times per hour. Does that rule take into account what fish gazer brought up (that, say, a 29 gal. tank is NOT 29 gal.)? What I'm saying is if you have a "29 gal." tank, do you want the canister turning over 29 gallons or 22 gallons the recommended 5-8 x per hour?
  12. renthusWell Known MemberMember

    I would assume that for all dosing and such, it's in terms of "per gallon of actual water". Honestly, having a dose that's dependent on the tank dimensions just doesn't make sense, if you think about it.
  13. EUFolk

    EUFolkValued MemberMember

    True. Dose/gallon is the right way you can always adjust to your volume.
  14. OP
    fish gazer

    fish gazerValued MemberMember

    I agree, well, sort of. I think what's even more important than their swimming habits it the surface area of the water. A 30 long and a 30 tall will indeed hold the same amount of water, and in general, yes, fish do enjoy swimming in a lateral direction.

    But a 30 wide can also support more fish than a 30 tall because the exchange of oxygen is reduced significantly in the taller one.
    Last edited: Jul 7, 2014
  15. OP
    fish gazer

    fish gazerValued MemberMember

    Q: When medicating or adding additives to our tanks, should we dose according to the purchase size of the aquarium or should we re-calculate the volume in regards to displacement and dose accordingly?

    Fluval (Hagen)

    A: “You just have to follow the instructions but it does not matter if you are just a little bit over the recommended dosage. You will not over dose with this product. If you have a 10 gallon tank then you give the recommended dosage for this size tank regardless if you have ornaments and such inside the tank.”

    Q: What do you consider “a little bit over the recommended dosage…”

    A: “We have heard back from our Aquatic department and you are right in saying that you should treat the volume of water you have in your tank.”


    A: “It is always best to dose for the actual volume of water, but in most cases, unless the tank has quite a bit of displacement, dosing for the volume of the aquarium will work fine. If you can accurately measure the actual volume, use that amount for dosing; if not, use your best estimate or the total volume of the aquarium. If you are dosing with Cupramine, some of the medication can be absorbed by porous substrates and decoration, so it is always good to use a copper test kit like the MultiTest: Copper kit to ensure you are maintaining the proper amount.”

    Tetra (found on their site)

    “In regards to dosing, it is best to not dose for the full amount of water as a tank will never have the full amount it can hold, due to the gravel, decor, etc, that displace some of it. In general, the amount of displacement can be anywhere from 5-10 gallons of water, in most cases. Saltwater tanks with live rock, etc, may be more.”


    No response yet.
    Last edited: Jul 8, 2014
  16. Adam55

    Adam55Well Known MemberMember

    In the big picture, I don't think the handful of gallons lost to substrate or ornaments really matters to the fish. Buy the largest tank you can find, put nothing in it except water, and it's still hundreds or thousands of times smaller than what they would have in a natural habitat.
  17. Waterhobit

    WaterhobitValued MemberMember

    Ok, the argument makes sense and technically you are correct, that there is actually much less volume than we think. But realize that all the estimations that have been made regarding inches to gallons, gph, and medication dosage were all made regarding the stated aquarium volume as opposed to the actual volume.

    So we could start dealing in actual gallons but then we should change it to the 1 inch per .8 gallon rule. And filtration can be measure in .8 gallons per hour. I don't know about you, but all of that is too much trouble, so I'll just keep it simple and stick with assuming that my 40 gallon actually holds 40 gallons.

    Personally, I like to think in terms of inches and rather than gallons. My rainbows have 48 inches to swim in. I think that is more important to them than gallons. Or maybe it is technically 46 inches, lol.
  18. OP
    fish gazer

    fish gazerValued MemberMember

    I agree, it is more annoying to figure it out and much easier to dose according to purchase volume, we could empty our tank water and measure the exact amount and dose accordingly or just estimate the new volume, but, ya. I have not decided which route I will take. So far I have been dosing to original volume. Sometimes, people add "just a little bit more" to make sure, but that's not good in this case!
  19. Christopher shoemaker

    Christopher shoemakerValued MemberMember

    Very good point!

    Sent from my SM-G900V using Fish Lore Aquarium Fish Forum mobile app
  20. Linda4088

    Linda4088Valued MemberMember

    How do I figure my tank size? I just got a used custom tank. The woman said she thought it was 35 gallons but the measurements are 36 long 12 wide and 22 high is that considered a 35 gallon tank?

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice