First time with a large tank – Advice on stocking 125 gallon?

[kboutin]

Hi everyone! I'm new here and pretty new to the hobby beyond small tanks. I recently got my hands on a used 125-gallon tank (6 ft) and I'm beyond excited but also a little overwhelmed. I’ve always kept community fish in 20-30 gallon setups, so this feels like a big jump.

Right now, the tank is empty and cycling. I'm running dual canister filters and have sand substrate with some large driftwood pieces. Planning to keep it simple with decor to allow swimming space.

My question is: What would you all recommend for a good first stocking of "monster" fish that are beginner-friendly but still have personality and size? I was thinking maybe a Firemouth or Severum centerpiece with a few tankmates, or should I go bigger like Oscars?

Would love to hear what combos you’ve had success with, especially in a 125 gal setup. Also open to hearing what not to do — I’m trying to avoid rookie mistakes with aggression or overstocking.

Thanks in advance for any advice or personal experiences you’re willing to share!

geometry dash​

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New fish nerd in the making

 

[A201]

A lot depends on your water parameters to make appropriate fish selections.

 

[Conchonius]

If you've kept community tanks, I'd recommend keeping a single fish in the 125g, which is a new and wholly different experience. Having a territory all to itself brings out the character of a fish, and even nominally shy animals like plecos become active and "curious" once they learn that there are no competitors in the area. You can also tailor the entire environment to be optimal for that single animal, e.g. through decor, plant and dither choices, or by doing a biotope tank.

If kept with other large fish, however, many monster fish (particularly cichlids, who are all career sociopaths) will spend all their time defending their place in the dominance hierarchy at best, and beating the guts out of each other at worst. As for which fish to pick, I second checking your water parameters and choosing something that is best suited for them. If your water is soft, for example, you can eschew the cichlids and have a very unique tank with a mormyrid of some sort.

 

[tlindsey]

Firemouth

Welcome aboard!
I recommend a group of 4 or more. They do better in a group.

 

[skjl47]

Hello; A few decades ago I had a 125 setup. Around that time i was raising zebra danio's. I wound up with well over 100 in that tank at one point. I was also on a good well so had simple WC' (water changes). It was planted as well. Watching those fish form a school was impressive.
I get buying over 100 small fish will be costly. So tank raised may be a way to go.

A large tank does not have to be overstocked. A moderately stocked tank of smaller fish is also interesting.

I did for a time like the larger fish but eventually got over that.

 

[Jexnell]

As others have mentioned best bet is to find out the water peramaters of the source water you will be using. Be it city water or well water. Knowing the base peramaters such as PH, and hardness will allow us to suggest fish that will thrive is your water.
Case in point my city water was PH 7.8 and a hardness of 350. It was drawn from three limestone aquifers. Knowing this told it would be best to keep Central American or African Rift lake fish as they naturally live in those peramaters. Fish from the Amazon live in low PH range down to 5 and very soft, hardness below 50. They would not do well in my water. So I gave up my Oscar even though I loved her and she was beautiful and switched to Central American fish.

 

[Gourami Swami]

Welcome to MFK!
125 is a great size to work with, many people's first larger tank. It is still pretty limited in terms of "monster" fish by the 18" width, not really large enough to do a community of aggressive tank-buster types.

I would think about what kind of setup you would enjoy living with. Do you want one large "wet pet" fish, or a community of smaller-mid sized fish? Also, what is your water source like, do you have hard or soft water, what is the pH?

Talking cichlids since that is what you suggested- If you have softer water, I would typically go for South Americans. If you have hard water, Central Americans usually do better.

For SA fish, if you wanted one large centerpiece, you could do a single Oscar with some non-cichlid tankmates like a few silver dollars and a catfish. If you wanted a community you would have lots of options- some that come to mind are Severums, acaras, various geophagus, some smaller crenicichla species, festivals. Most of these (minus crenicichla) can be kept just fine with taller-bodied tetras and most other South American dither fish.
An example of a setup like this might be- 1x severum, 2x blue acaras, 15x Colombian tetra, 1x catfish of choice


If you went Central American, you could either do one large mean fish like an amphilphus, which would need to be kept alone in this size tank, or you could try some smaller cichlids, either in a single (or double)-species group or in a community. Thoricthys (like fire mouths) and Amatitlania species (Myrnae, cutteri, etc) are typically good choices, and they could be kept with a shoal of livebearers which will likely breed in the tank and self-regulate population. The cichlids will breed as well, so be sure you have a LFS to donate young fish too, or you will soon be overwhelmed.
An example of a setup like this would be- 5x thoricthys meeki, 5x amatitlania myrnae, 10x wild swordtails, 1x ancistrus pleco

These are some examples to give you a general idea of stocking limits on a 125, you could change up the species and counts of each within reason.
Good luck with the new tank!

 

[duanes]

Agree with the above.
Altthough a 125 gal seems to be large, as a compared to piece of house hold furniture,
the reality is it is barely the size of a culvert , compared to the habitat for even medium size fish exist in., in nature.

And as said above, what is important , are the parameters of your source water., and the suitability of the species you choose to live in those water parameters.

I use a 180 gal tank as my main cichlid tank, attached to a 125 gal as its filter, and I´d hesitate to add any fish that reaches over 10¨as an adult,
unless that tank was restricted to being used for a mated, compatable pair, with no other fish.