New to Forum - Desperately need help


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Dec 26, 2019
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Hello everyone!
This may be a bit of a lengthy post, but I desperately need some help, advice, telling off, whatever the solution is to my problem...

So I caught the fish bug a couple of years ago, and have upgraded tanks a couple of times during this time. I actually won my first aquarium on a competition, the fluval edge 21l. I kept just 3 mollies. This kicked off my fish bug, I then later upgraded to a 80l tank added a few more fish.
Then I moved house with more space and upgraded to a 250l aquarium.

My aquarium has been up and running for 6 months, I transfered my mature filter medium from my old tank to my new set up, and had no issues.
For this first 6 months I kept just my fish I already had from my 80l aquarium without adding any new stock;

3 Regular Mollies
3 Sailfin Mollies
3 Panda Corrys
Some Molly fry.

All has been fine until about 1 week ago.
I thought right it's time to add some new fish! So on the 18/12/19 I bought 6 cardinal tetras and a king tiger pleco.
All was well.
On the 19/12/19 around 1:30am we had a power cut, when I got up at 6:00am the power was still off, one molly had died.
Power came back on and off until around 09:00 when the issue was fixed.
Obviously the temperate of the aquarium dropped really low, it was reading 18c when I got up.

Since then my mollies have slowly died one by one, pretty much 1 a day until this evening when my last adult molly died.
I only have 2 small baby mollies remaining and I am desperate for them to not suffer the same fate.

One molly that died was lay on the bottom on the side before it died
A second molly that died seemed to have lost its bouyency and when it tries kept swimming into objects in the tank
A third developed 'pop eye'
A fourth developed a dark patch on their belly.
The others were just found dead.

The other fish in the tank seem to be unaffected as (touch wood) had no losses:
3 Panda corys
6 Cardinal Tetras
1 King tiger Pleco

I have an API test kit which I use to test the water parameters, Ammonia, Nitrate, Nitrite, PH level.

Ammonia 0
Nitrate 0
Nitrite 0
PH 7.4
Temperature 26

The tank is planted, gravel substrate, drift wood, rocks.

I have tested the water over and over. I really don't know what to do.

I don't know if it's been caused through the power cut or the new fish. So I have been medicating with pimafix and melafix

Any help would be greatly appreciated, and any idea as to what has caused this would be massively appreciated

Hi Ashley,

Whenever you add new fish, usually there is a high risk of bringing diseases into your tank. But since your Cardinal Tetras and Pleco are doing fine, then it might not be due to disease(parasite or bacteria infection).

The only reason that I can think of that killed the Mollies is the power cut off.
During power cut-off, a few things might happened that killed the Mollies:
1)Lost of filtrations temporarily - ammonia and nitrite will rise and kill the fish especially for crowded tank.
Quickly perform large water change - at least 70% of water if you suspect that the ammonia and nitrite is high.
Or some beneficial bacteria might had died during the power cut off.

2)Low level of oxygen as there is no water agitation from the filters.
Your tank is quite crowded now and any drop of oxygen may kill the fish especially bigger fish.

3)Drastic drop of water temperature will kill the fish.

I have some questions:
1)How big are your Mollies that died?
2)What kind of filters that you use?
3)How frequent and how much water you change each time?

Here is a very good article that explained why fish died?

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The Mollies that died range in size the smallest being 1/2 an inch to the largest being about 2 inches. They didn't die in any particular size order.

I use an eternal canister filter, tetra ex 800.

I do a weekly partial water change of 25%

I tested the water after the power cut and after every death, looking for an explanation - but there was never a spike in any reading.
I don't feel like my tank is crowded for oxygen levels to drop enough to kill the mollies, especially a week after the power cut?
The Mollies that have died have died over the course of a week, pretty much one daily
Then most likely your fish died due to drastic temperature change.

See the article that I posted above.

Here is another discussion from the past.
Read the reply from a guy name "Wilder":
He mentioned the fast temperature drop is the cause.

The other thing to ask is how hard is your water? Mollies are hard water fish and if kept in soft water they will have been weakened meaning that something like a temperature drop would affect them more seriously.
Cories and cardinals are soft water fish, so you do need to find out the hardness as either the livebearers or the tetras will not be happy. Look on your water provider's website - you need a number not some vague words.
I agree with @essjay. I typed this earlier and forgot to hot the post button...

I agree that the temperature seems the most likely cause.
What is the hardness of your water? Mollies require hard water and all of the others you mentioned are soft water fish. These would not both thrive in the same water. If your water is soft it may be that your mollies have been weekened over time and were less able to cope with the temperature drop.

You should check this info and take it into account when you re-stock. The number should be available on your water provider's website. We need the number and unit for GH.
Unless your tank is heavily planted. You should be showing some nitrates. Are you sure your tank cycled?
Another thing to take note is that when you bring in new fish, the new fish could be a carrier to some parasites/bacteria.
Though the new fish may be immune to the parasites / bacteria, your other fish may not.

So, continue to observe for any symptoms of disease for another 3-4 weeks.

But I suspect the main culprit is the water temperature dropped too low and fast.
I think the culprit is an uncycled tank.

It's hard to say because he mentioned that he is having his current tank set-up for 6 months. He is using a canister filter and his tank of 250 liter(4 by 2 feet tank) is more than enough for the light bio-load.
Probably his measurement of nitrate is not so accurate.(just my guess).
Let's wait for his reply.
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