tank dying off-- help!

[bobbme01]

Hi guys- long time lurker first time poster.

I have a 20 gallon (tall) planted tank that has been established for 5 years with usually 5 pearl danios, 6 cardinal tetras and 5 cory catfish + 3 nerite snails all happily coexisting. This was my first ever tank- made a lot of mistakes at first but have learned a ton over the past 5 years.

I was down a few fish and I hadn't replaced so went to try out a new, reputable fish store (instead of my normal "petsmart" to restock) about 2 weeks ago- put in 5 tetras and 2 cory cats to restock back to appropriate schooling numbers. I also added a new live plant. Did everything the same as I normally do- continued 15% water changes + addition of live bacteria once weekly, feeding once daily, let the fish acclimate to the water prior to putting them into the tank, clean hands and fish tank-only bucket etc.
One of the tetras died within a few days of putting them in the tank- didn't think much of it.
A week later- I had 5 dead dish all in the same day- all of my tetras including the preexisting one that I've had for well over a year.
All of my other fish seemed totally fine at this point. I did a 50% water change out of panic because I've never lost that many fish at one time before and added a bunch more bacteria. Ordered new parameter test strips because I haven't checked my water parameters in years because my tank has always been stable.
Since that time- I've started losing my cory cats. I lost one that I've had for four years as well as both of the two new ones. They all acted and looked completely fine as far as I can tell in the day leading up to their death. One of my remaining cory cats is acting sick- swimming like its his swim bladder but doesn't look bloated.

Water Parameters:
Ammonia, nitrites, nitrates 0
pH 8.2 and baseline hard water- these have been like this since the beginning, I have peat moss and driftwood in the tank and have never gotten it lower/the fish have usually tolerated it.

Is this a parasite? I don't have a quarantine tank cycled so I don't know what to do or how to save my remaining cory cat and danios. I'm also terrified to restock and have no clue how to move forward.

thanks everyone

 

[Colin_T]

Hi and welcome to the forum

Pictures of the fish so we can check them for disease?

What sort of filter is on the tank?
How often and how do you clean the filter?
What filter media/ materials are in the filter?
Do you ever replace the filter media?

Do you normally only change 15% of the tank water each week?
Do you gravel clean the substrate when you do a water change?
Dp you dechlorinate the new water before adding it to the tank?

What is the GH (general hardness), KH (carbonate hardness) and pH of your water supply?
This information can usually be obtained from your water supply company's website (Water Analysis Report) or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).

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BASIC FIRST AID FOR FISH
Test the water for ammonia, nitrite, nitrate and pH.

Wipe the inside of the glass down with a clean fish sponge. This removes the biofilm on the glass and the biofilm will contain lots of harmful bacteria, fungus, protozoans and various other microscopic life forms.

Do a 75% water change and gravel clean the substrate every day for a week or until the problem is identified. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in. It also removes a lot of the gunk and this means any medication can work on treating the fish instead of being wasted killing the pathogens in the gunk.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.

Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens so any medication (if needed) will work more effectively on the fish.

Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.