I have a community tank with a mix of cichlid species ranging from malawi to central america species.

The tank is 102 Gal (150x60x60) and there have been no fighting to date, even with some of the species traditionally classified as agressive (there are sufficient hiding place / structure etc).

What is the max number of fish? Is there a difference for the Malawis vs American species such as Green Terrors and Green Texas?

Hi

A list of the fish you have (species and number of each) plus water test results for ammonia, nitrite, nitrate, hardness and pH are essential to answer the question properly. Cichlid stocking can't be calculated by a simple formula as there are too many compatibility/bioload issues to consider.

One vital thing to note is that Malawis and cichlids from other regions should not be combined in the same tank.

Part of the question is due to the fact that I'm battling with the stability of the tank. one day pH will be fine and the next it will be extremely acidic. (I've started to use buffer, which seem to be working).

We have the pH currently at 7.2, the GH at 140 and the kH at 80. Ammonia and Nitrites are at 0 and Nitrates range between 25 and 45.

We have a cannister filter as well as a hang-on filter on the tank with Gravel substrate and two additional air lines.

Fish stock currently:
Parrot x 2 (large)
Green Terror x 2 (large)
Fire Mouth x 2 (Small)
Green Texas x 2 (juvinile)
Peacock x 3 (roughly 8 cm ea)
Peacock x 4 (juvinile)
Rainbow x 2 (7 cm)
Oscar x 2 (juvinile)
Malawis x 8 (all ranging 4cm to 7 cm)
Pleco x 4
Jewel x 3 (+/- 10 cm)

Hi

Unfortunately the stock you have is going to be a serious problem sooner or later. You have fish from a variety of different geographical locations who are not compatible due to a combination of differing water requirements, dietary needs, aggression levels etc. Many are juveniles so territorial problems haven't yet arisen, but they will as the fish mature. The tank is also overstocked, and some of those fish will outgrow the tank quite rapidly.

On the stock side of things we would normally stock 1 inch per gallon (at projected adult size) on a newly cycled tank, gradually rising to 2 inches per gallon as the tank matures, if all indications are good. This formula doesn't work very well with large/heavy bio-load fish - they usually need more volume, and with aggressive fish who can have complex territorial needs .

However using that rule as a very rough guide your tank is overstocked even by that standard:

Parrot x 2 at 18 inches each = 36"
Green Terror x 2 at 12 inches each = 24"
Fire Mouth x 2 at 6 inches each = 12"
Green Texas x 2 at 12 inches each = 24"
Peacocks x 7 at (rough estimate) 5 inches each = 35"
Rainbow x 2 at 6 inches each = 12"
Oscar x 2 at 13 inches each = 26"
Malawis x 8 at (rough estimate assuming dwarf Mbuna) 5 inches each = 40"
Pleco x 4 depends on species, but up to 24 inches each = 96"
Jewel x 3 = 6 inches each = 18"

The tank is 102 gallons, so about 90 gallons of water once rocks/gravel etc are taken into account. By the "inch per gallon" rule it will hold 90 inches of fish, rising to an absolute max of 180 inches. Your stock adds up to 227 inches excluding the Plecos - if they are Common Plecos that takes it to 323 inches So, extreme overstocking is the first problem.

Next, you have fish from different areas who need different water hardness and pH:

Lake Malawi - hard, alkaline water: Peacocks and Mbuna.

South America - soft, acidic water: Plecos, Parrots (they are questionable as they are hybrids from SA/CA fish), Green Terrors, Oscars, Texas Cichlids.

Central America - harder and more alkaline than SA, but not so much as Malawi: Rainbows, Firemouths, Jewels.

Beyond that there are further issues, but as you are going to have to rehome the fish that don't suit the water, the first thing to do is test the gH, kH and pH of your tap water supply. Please test it twice - once immediately it's drawn from the tap and then again after it's been standing in a clean glass or cup for 24 hours. Post the results and we'll look at which of the fish are actually suited to your local water.

Hi Fishi

Like FL said, it's impossible to mix a tank like this. You'll have to decide which type of cichlid you like the best and keep only them. If you don't act fast your fish will have to suffer a lot.

I guess you was taking advice from local fish shop?

With big messy fish like Oscars and Plecos following the "6x the body length" rule works better than the 1" per gallon in deciding if a fish is going to be suitable for your aquarium. Generally people have relatively few individuals in such tanks- two or three fish for example.

I don't think your tank is big enough for any of the South American Cichlids you have. Oscars get to over a foot, and the others are similar sizes, so your five foot aquarium isn't big enough- if I were keeping them I'd want at least a six foot tank for a matched pair, and the same tank again for each pair of Green Terrors, Green Texases, etc. 6 12" aggressive fish in a 5' tank is not going to end at all well. Especially if they pair off.

Depending on the personality (and water preferences) of the cichlids you might be able to keep a catfish with them. According to the 1"/gallon "rule" you'd be understocked, but in practical terms you wouldn't, as you'd be giving the fish ample space to move in, and a 12" fish is 8x larger in volume than 12 1" fish (and hence has significantly more bioload).

Depending on your water parameters, the smaller Central American Cichlids would be better suited to your current tank I think. But even then I wouldn't mix them all together. I'd keep a single species- Jewels are the most aggressive, followed by the Rainbows and Firemouths which are both relatively relaxed by cichlid standards and then you'd have plenty of stocking capacity left over for some midwater fish. Rainbowfish would suit the harder water that the CA cichlids like.

If your water is very hard, I would convert the tank into a Malawi set-up.

Regardless of what you do virtually all your current stock will have to be rehomed.