Fish dying one by one - Bacterial infection?

[Gyyome]

1. Water parameters.
ammonia = 0, nitrite = O, temp = 24 celsius
2. A full description of the fishes symptoms.

We're compete beginners and set up a new tank 3 weeks ago. We've cycled the tank prior to introducing any fish, using filter media from another aquarium. Then we introduced 9 guppies and 3 mollies. Since then we've lost 4 guppies.

The first guppy was found dead after a day. We just figured that was bad luck.

Second and third guppy spend 1 or 2 days swimming at the surface, gasping for air it seems.

When a fourth guppy started behaving the same way we figured we had a lack of oxygen in the tank. We've added an airstone at that point (for some reason never encountered the need for one in our research). He seemed to be doing better for a couple of days. Then he started swimming close to the surface again. The final day he was sitting at the bottom before finally dying. I did notice a red spot on both sides near the gills that wasn't present on the other guppies.
Now we have a molly that's just floating in place, sometimes at the bottom, now closer to the top. She also seems to have paler lips than her mates. Pics for comparison. She hasn't eaten anything at feeding time today.

Sick molly:

https://imgur.com/L8BdIxM

Normal molly:

https://imgur.com/cwAKaH7

3. How often you do water changes and how much.
Weekly 25%

4. Any chemicals and treatments you add to the water.
Aqua start

5. What tank mates are in the tank.
Guppies and mollies

6. Tank size.
20 gallons

7. Finally Have you recently added any new fish?
It's a new tank so all fish are new.

8. Other remarks
I have noticed a strange white substance of one of the fake plants.
White residue in middle of fake plant:

https://imgur.com/VftyrgO

Given what I've read on this forum up until now i'm suspecting a bacterial infection of some sort?

 

[Byron]

My first thought is a "cycling" issue, but if ammonia and nitrite are zerothat is less likely. Do you know the nitrate level?

Related, what is the GH (general hardness) and pH of your tap water? And tank water for comparison?

Diagnosing fish ailments is very complex for most of us, and removing possible causes is often the best way to find the actual cause.

Welcome to TFF.

 

[BrianK]

Are you using something to neutralize the chlorine/chloramines?

 

[Gyyome]

I don't have a test for GH unfortunately.
Nitrate = 5 ppm (thats after a 50% water change I did yesterday)
PH of tankwater = 7.5
PH op tap water = 8


One symptom I forgot is that 2 of the dead fish produced white stringy poop the days before dying.

 

[BrianK]

white stringy poop is usually a parasite infection

 

[Byron]

A couple observations on the parameters...a lowering pH in the tank suggests either a low GH/KH, or they are adding something to increase the pH but it is not permanent. Check the website of your water authority, they may post the GH and KH (also called Alkalinity). The source water GH and KH is worth knowing. Also, see if something is being added to increase the pH.

Also, BrianK questions are very pertinent.

And, what exactly is "Aqua Start?" I could not find this searching online.

 

[Gyyome]

Aqua start => https://www.pro-shrimp.co.uk/water-...rt-250ml-water-conditioner-8715897225406.html
It detoxifies and declorifies the tap water. I use it for every water change.

KH of the tank water is 8 by the way.

 

[Byron]

OK on the Aqua Start.

The KH of 8, is this degrees or ppm (parts per million, which is the same as mg/l)?

Can youfind the GH?

 

[Gyyome]

The KH is 8 ppm. That's of the tank water.
I've just tested the tap water. This gives me a KH of 10 ppm.

Can't find any values on the water supplier's site regarding GH unfortunately.
Just a note on the PH test I'm using; It goes in 0.5 increments, so it's not terribly precise. The tap water could realistically be anywhere between 7.5 and 8.0.

 

[Byron]

OK, I was working on the parameters because these impact fish. The GH primarily, due to the dissolved calcium and magnesium (needed by hard water fish, not needed by soft water, generally), and then the pH. The KH is important because it serves to buffer the pH more. At 8 or 10 ppm, it is very low. And this usually corresponds to the GH, so assuming this to be the case here, the GH is probably soft and will significantly weaken the mollies especially so they are more susceptible to all sorts of diseas/health issues. The guppies are more tolerant, but if the GH really is as low as I am thinking, they won't do well either.

So back to the initial issue...the GH may well be the root of the problem.

 

[Gyyome]

Woops, looks like I misread the test's documentation.
The KH results is in fact degrees NOT ppm.

So KH of 8 degrees for the tank water and 10 degrees for the tap water.
Apologies for the confusion.

 

[AdoraBelle Dearheart]

Something to consider - Fish store livebearers often come loaded down with worms (which stringy white poop suggests, but doesn't usually kill so fast without other visible symptoms - they can carry a worm burden for a long time) but also that many fish farms abroad often use sea water to help fill their tubs, meaning the fish can be used to brackish water, and have a hard time adjusting to being in fresh water.

There are also often diseases rife in the fish farms and in store tanks. I've found fish store guppies to be very weak in general, and had a lot of losses when i started, despite water testing perfectly, lightly stocked, fully cycled tank packed with live plants etc. I learned that if you buy livebearers from a store that likely came from a fish farm abroad, that you count yourself as lucky if you get some fry from them before they go belly up, and that the fry will usually be hardier than the parent stock.