Harlequin Query

[Silvertabby]

Hi, I was wondering if anyone can help me?

I have (well, had) a shoal of Harlequin rasbora. Then recently, they all started dying off one by one. I had a brief bout of finrot after my Betta attacked them (he's now in a separate solitary tank), which was treated with Melafix and went away. But the same symptoms keep coming back and killing off my harlies.

The symptoms are: they go very pale, and start lurking at the bottom of the tank by themselves. They then start twitching - if they were mammals I'd describe it as a minor epileptic fit. It doesn't look like they are itchy, they don't rub against anything. The one I've seen actually die started flinging himself around the tank as if he was having a major fit, bashing into everything. It's like he lost all control of his muscles.

This hasn't affected any of my other fish, just the harlequins. In the last months I've lost 6. Anyone know what this is?

I've ruled out water problems, my tank is fine for ammonia, nitrites and nitrates. It's only 21L, but isn't overstocked. pH is fine,temperature is 25 degrees and constant. I have 2 air stones running, live plants and a small bogwood.

 

[the_lock_man]

When you say "fine" for ammonia, nitrite, nitrate and pH, can you tell us what the actual levels are, please?
 
A 21l tank with a shoal of harlies plus others is rather overstocked, sorry to burst your bubble on that one. The stress of this might be contributing to the problem. Did you add anything new to the tank just before the problem started? What is your maintenance schedule? What chemicals do you use? How was the tank/filter cycled?

 

[Silvertabby]

Ammonia and Nitrite were at 0. Nitrate at 40. pH I believe was 6.8 at last check. Temperature is a constant 25 degrees.
 
I say 'shoal', I haven't had more than 4 at a time in there, with 3 Cardinal Tetras (once the harlequin numbers reduced, I upped them to 6 and it made the Tetras happier). Current numbers are 2 Harlequins and 6 small Cardinal Tetras.
 
The main reason I am asking, is not because I want to add more Harlequins again but because I want to be certain this will not also kill my Cardinals or anything else (I have 2 Yamoto shrimps, 3 nano shrimp and an assassin snail (plus pest snails) in there with them).
 
The initial thing that triggered the first harlequin getting upset was the introduction of a Betta, about a month ago. At the time, there were 3 Cardinals and 3 Harlequins. He stayed in the tank a grand total of three days, before being moved out to his own 12L tank. It would appear I acquired the worlds stroppiest Betta. I also used the opportunity to do a complete clean of the gravel (netting the fish into a bowl temporarily, in their old water), and I'm told that may well have stressed everyone out a lot. However, since then Harlequins that never even saw the Betta nor were in there pre/post clean have died.
 
The tank has been cycling since April 10th this year. I use Interpet Bioactive Tapsafe to dechlorinate the water. Once a month I add Tetra Plantamin for the plants. When adding new fish or doing a larger water change I use API Quickstart. After smaller water changes I add a dose of API Stresscoat+. I feed them every other day, with the air and filters turned off for about 5 minutes. When the first Harlequin died, the symptoms of his beating from the Betta seemed to match fin rot so I did a 7 day course of API Melafix. I do a routine water change of about 20-25% every sunday, though on advice I have been doing a smaller gravel clean and 10% change every few days to try and dilute anything in the water they don't like. Water is going in at close / slightly cooler than 25 degrees. There is no lighting rig in the tank (I tried, but it terrified them as anything I got was far too light for the small tank) but they have good ambient light from a non-direct sunlight window next to the tank.
 
Any upgrading to a bigger tank will not happen til next year, due to lack of room and money. I know I've made a lot of rookie mistakes, but I was hoping things would settle once it was all cycled in and yet the fish just keep dying... Both myself and the store staff, and all my fishkeeping friends are at a loss as to why the Harlequins keep dying...