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​

---

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.