Going to be overpopulated...


New Member

New member, long inactive aquarist just getting back into it due to a curious daughter I don't remember it being this much work lol!

We started with a 10 gallon housing 4 mollies and a couple of otocinclus. The darned mollies are like a couple of randy teenagers breeding very rapidly and we quickly realized a tank upgrade was in order. We had established a 29 gallon tank and migrated the fish to that (at that point about 8 young mollies, the 4 adults plus the two otos). All seemed well enough so we introduced a couple of Albino Cory's(planning to introduce two more based on suggestions I've seen that these little guys like to cruise in groups of at least 4). About a week later we had a major algae bloom and I was a little overzealous in cleaning the tank, likely wiped out too much of my good bacteria resetting the N cycle and saw massive spikes in ammonia, nitrite and nitrate levels. We quickly moved the fish back into the 10 gallon that I had left running just in case while the 29 re-established, but lost a couple of the adult mollies before realizing my over cleaning mistake.

I can't find any LFS that will accept fish...well more like I can't find any mom and pop LFS that would do so, they are all out of business and I am stuck with a Petsmart. Not having much luck among friends and family finding new homes either so I am left with the choice of euthanization or trying to figure out how to support the excess population with what we have.

My longterm plan is to introduce some black skirt (black widow) tetras to start eating up any new Molly fry, however they will most certainly push my tank to way overpopulated. I would really rather not euthanize. As a young aquarist I would have done so without hesitation but a 9 year old girls tears will prevent me from doing so now

If I introduce the tetras and the aforementioned two additional Albino Cory's my population will be something like this (note my female lyretail molly had a second and appears ready to pop out a third batch of fry):

5 Black Skirt Tetra
4 Albino Cory
2 Otocinclus
2 Adult Lyretail Molly
~8 very young mollies (less than .75 inch)
~8 juvenile mollies (.75 to 1.5 inch)

I am planning to add a Fluval 110 which will replace or maybe run alongside my current Tetra Whisper 30. According to AQAdvisor, with the Fluval 110 alone it can support this population at current sizes but the youngsters are obviously going to grow to full size (or most of them anyway) and I will be close to 200% capacity assuming AQAdvisor is to be trusted. If I work the numbers it appears that around 8 adult mollies with the same numbers of the others will be within the limits of my environment, but I am going to have more like 20!

Is my only option going to be to get rid of some of the excess mollies? Is there anything can I do to increase the population capacity without going to an even larger tank? Preferably something that will not involve daily testing and frequent water changes (i.e. not labor intensive). I understand the Fluval filters allow for additional media. Is there something special I can use along those lines to control ammonia levels in order to support the larger population?

Sorry my tank specs are not in my profile. I cannot seem to edit it, I am told that I must be an active member to do so, will try again after I post.

Tank spec:

29 Gallon Tall
Tetra Whisper 30 (soon to be running alongside or replaced by a Fluval 110)
3 Bubblers - bubble disc, small fine bubble stone and treasure chest with a regulator valve set to low output
All plantings are fakes, 6 pieces of varying sizes
Temp = 77-78 F
pH holds steady around 7.0 with little effort
Nitrates at 20-40 ppm (stick test)
nitrites at 0 (stick test)
occasionally get a blip in ammonia of about .25 ppm (liquid test kit) between PWCs indicating I am likely pushing the population as it is I would assume
Kh = 80 (stick test, a little low)
Gh = 120 (stick test)

yes I know I need to get liquid testing kits instead of sticks. I'll be picking them up this weekend along with the second higher capacity filter

Thanks in advance and sorry for the wordy post!


Fishlore VIP
HI welcome to fishlore.

Since you don't want another tank, and the mollies will simply keep breeding then yes you kind have to get rid of them. It's also very stressful on the female molly to keep having babies back to back to back. I would rehome her as well.

I understand it's difficult with a child, I have two of my own, but you can make it a teachable moment with her on how to properly care for fish.


Fishlore VIP
Agree with above, when keeping a lot of fish over a tanks biological limit and humanely keeping quality of life your gonna need intense labor such as water changes and vacuuming.

So mollies are huge on bioload and the max I would say for a 29 gallon would be 6 that is if they are the only fish in the tank and you have a gph of 300. Also the filter you have now is too small to process the ammonia given off your fish. A filter size for a tank is judge by the gph not what the box recommends. Most of the time what the box recommends is actually too much thus your getting a smaller filter for your tank. With HOBs you want 8-10x the gph of the size tank, however with mollies your gonna want 10 or more. When the mollies get bigger the ammonia will rise.

The best thing to do is separate the mollies between male and female until you can find new homes. Females can store sperm for up to a year and reuse the sperm 2-4x.
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New Member
Thank you both for the quick replies. Kind of figured that would be there answer short of a lot of work...

I am inclined to keep the males, they tend to be more colorful and have cooler fins. Is there any risk of increased aggression towards my Otos or Corys if I start plucking out the females? Should I quarantine the males for a few days until they get used to it or can I just take the females right out of the 29? There is really only the 1 adult male right now, though the bigger juveniles sure seem to know what their special fins are for I still have the 10 gallon running with pristine water conditions that I can use as a temporary home either way.

@ Al - yes I am aware my filtration is way below what I need, that's why I am upgrading to the 110. It moves 500 gph which should offset the excess bioload from the mollies. They are among the dirtier fish, like goldfish & guppies, according to my research. Were I not subject to the whims of a 9 year old they probably would not be in our tank The fluval 110 has what I perceive to be better media options from the other products I have looked into (both cost and diversity), though I am open to suggestions if you believe there is a superior HOB that won't set me back a fortune and has relatively low cost of upkeep (media supplies). I was originally looking at one of the mainland tumbler wheels but there was a lot of negative feedback out there on their products (noise and parts breaking).

Thanks again!

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