Questions about number of fish and tank size

ScubaSkeets · Jan 10, 2021

Hi folks!
When looking at information for a fish that i may want to buy, it usually has a minimum tank size for that fish. I understand all fish have different requirements, but, for example, according to Liveaquaria, the Yellow Tang's max size is 8" Liveaquaria has a minimum tank size of 100 gallons for it.
Now, I have a few questions.
1. Does that mean one Yellow Tang per 100 gallons? If I want two, does that mean I need a 200 gallon tank. Can I not put anything else in there with it? Or is it because that particular fish just needs that much room to be able to swim more than it could using a say, 50 gallon tank?
2. I realize the one inch per gallon rule of thumb is flawed, but if I were to hypothetically follow that, that would mean I could put that 8 inch Yellow Tang in an 8 gallon tank (no, I would never do that...just a hypothetical). However, even if I used a one inch per 5 gallons calculation, that would mean I could put that 8 inch Tang in a 40 gallon tank, which, again, I would not do. However, at least in theory,, a 40 gallon tank should be plenty for an 8 inch fish.. So why does that particular 8 inch fish need 100 gallons of water when a different 8 inch fish might not necessarily need 100 gallons. I realize that different fish have different swimming habits/needs, but is that it?
3. Using the hypothetical one inch per gallon, if I were to have a 40 gallon sump tank, does that (hypothetically) mean that I could add "40 more inches" of fish to my display tank, considering size of the display tank?
4. Is one 8 inch fish the same as two 4 inch fish, or four 2 inch fish in terms of size requirements?
5. What other thing should I consider when thinking about the number and size of fish? What "rule" do you follow?

Thanks!
Ed

JosephM · Jan 10, 2021

I can’t answer all of those but I can share some information of what I have learned. Adding a sump does give you more room for bio load of fish but the issue with a lot of saltwater fish is more aggression then anything. They have their own territory in the tank. Many fish will attack another fish simply for having the same color as them. I have a smaller tank so I have to be a lot more careful then someone with a larger tank. I’ve read that a 1 fish per 10 gallon. Every fish has their own personality as well so sometimes it is just trial and error. You could have one fish of the same species that is docile while another one of the same species could be aggressive. I have a 29 gallon tank with a 20 gallon sump. I don’t follow any certain rule but more so think my stocking out more in terms of general aggression of that species, where in the water column they normally stay, and the color scheme of the fish. Sorry I don’t know more and can’t answer more but I’m still pretty new to the hobby.

Everclear · Jan 10, 2021

From what I understand, the (flawed) inch per gallon rule is more in reference to bio load in the tank whereas the suggested tank size is in reference to how much room a fish needs to swim and live in.

MoKan Reefer · Jan 10, 2021

I pay little to no attention to the "rules" of fish when it comes to my tank in all honesty. I did when I first started out in the hobby ~15yrs ago. I don't anymore because I've realized they're basically bs. I have 7 tangs (2 purples, blue hippo, naso, and 3 tomini) in my 180 along with multiple blennies, gobies, 2 hawkfish (only fish regret so far) and lots of invertebrates. All tangs are over 5". Same fish I had in my 150. When I move to the 270 which will be a foot wider, I will be having my friend who owns an lfs order 5 yellow tangs, 2 goldrim and an Achilles which I'll be quarantining and will be introduced at the same time as the fish I currently (along with all my current rock and biological filtration to prevent a huge nutrition spike and cycle). That way noone has time to establish any type of territory. They'll sort things out on their own. The tominis I added at the same time and had no issues. I expect some squabbles with the yellows and purples of course though lol.

Now as for tank size to number/size of fish, use common sense and do your homework about the max size of the fish is my advice there lol. Don't add 5-10 fish to a new tank that isn't mature enough to sustain the added nutrients. Be smart. I had 25 or so fish without issues in the 150 but they had been added over time and it had been up for years and none were excessively large fish. My thoughts is if you aren't like some freshwater keepers everyone has seen with an 18+" arowana along with 10 other 10+" fish in a 180, you're good.

Be realistic and smart about what your tank size and just and just as importantly the filtration not only the age of your system the amount of biological filtration it's capable of sustaining but the mechanical filtration. Skimmers are for the most part grossly over estimated to be honest. If you don't have filtration capable of removing the amount of nutrients your fish are producing, you're going to crash your system. Simple.

Also do a bit of research on specific fish nutrition and temperament. The 2 hawkfish mentioned above were mistakes because I knew hawkfish were considered reef safe and peaceful but apparently not all are either. So now I get the fun of evicting them as I replace my rock scape here soon because I made the mistake of adding a arc eye hawkfish on a whim who has a taste for smaller shrimp species and both hawkfish are aggressive as can be and kill any fish large or small I've tried to add since them. Not researching 2 $20 fish has cost $60 in shrimp (10 $6 peppermints) and a ton more in fish (killed 2 goldrim tangs, 2 aiptasia eating filefish, and 6 lyretail anthias). Costly mistake and lesson learned.

Anyway, just my 2 cents worth on stocking.

ca1ore · Jan 10, 2021

You have shown quite nicely how singularly useless the old inch-of-fish-per-gallon is. The things to consider when picking fish are swimming room, compatibility between fish and nutrient management. Swimming room is a function of both tank size and the aquascape. I typically want a tank that is as long as 10x the length of a fish. So, for a yellow tang that probably maxes out at around 6” in an aquarium, that means a 5’ tank. There are some exceptions. An Achilles tang is a swimmer, so needs more than 10x. Damsels establish micro territories, so less than 10x.

Multiples is more about compatibility and nutrient management. You could certainly put two yellow tangs in the aforementioned 5’ tank from a swimming room perspective, but do you have the requisite nutrient management capabilities, and would they tolerate one another?

Keep in mind that volume increases disproportionately with length; so a single 8” fish will be the biomass equivalent of more than two 4” fish. I forget the math and am too lazy to look it up, but I think you get the point. I generally assume half the length is a quarter of the mass.