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Confusion With Introduction New Fish

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    Hi I was wondering if anyone could give me some advice.

    Its probably best i give some background information first, I have a Juwel Rekord 96 tank which is a 96 litre tank. It was a successful and well balanced tank until it got a disease which i suspect was from a batch of live food I bought. This lead to the majority of fish dying leaving just two dawn tetra and a rainbow shark, which all seem healthy. I left the tank for a couple of months to make sure it was all clear and to check that the fish that remain were healthy, after cleaning it thoughout. I then introduced a group of six Harlequins which unfortunately died fairly quickly, within a few days, which brings me to my first question and point if confusion…

    How can i have a tank with three healthy fish and then when i put in what i assume are healthy fish bought from the local shop all the new fish die, just leaving me with the three original fish which all are still healthy. To the best of my knowledge the water temp and chemical make up are all as they should be, i’m also following all the correct instruction for how to introduce fish, so I am lost to why they died? (I know introducing six fish at a time is a fairly large amount but as i understand with shoaling fish six is really the minimum group they should live in.)

    I have noticed my shark is sometimes aggressive could this cause a large amount of stress to the small fish? and if so how can i get round this as most new fish will be small and i have a shark in the tank already.

    I was looking at a combination of any of these fish in which to introduce to the aquarium once i have solved the above problem, could anyone give me advise on which are best way to introduce and in what order bearing in mind what I have already.

    Possibly a couple more dawn tetra, Harlequins, Cherry Barbs, Neon Tetras and possibly a Pictus Cat although I will have to research this further as i know very little about this fish. I understand that my tank is limited to around 20 fish because of its size with each larger fish accounting for about three of the total.

    Thanks for your help.






    Mark’s right Cmoss, the other option being that the harlequins had some kind of internal infection. I definitely don’t think your tank is big enough for a full-grown rainbow shark (too territorial) or P. pictus (grows too large, does far better when kept in a group, will probably eat your small fish) either. Sorry for the bad news but hope that helps. Welcome to the site by the way!




    I try not to change the water more than necessary as I know it can damage the aquariums ecosystem, usually changing no more than half. I dont do this just before I put new fish in, i would have probably changed it over a week before the new fish went in, in this case.

    So would it be better if I had changed the water more recently when adding new fish, so the condition of the water are similar to that of a shop?

    Thats a shame about the Rainbow shark, ive had it for a while, it was one of the first fish I bought so didnt know much about them at the time. Its not quite fully grown (It may be for the tank of my size) at around 3-4inchs, but ill defiantly introduce new fish carefully knowing what i do about them now.

    If I was to introduce a group of shoaling fish is it best to do it in a group of six or is this a lot for a tank of my size?

    Thanks for the advice about the pictus, I think its a fish I may have one day in a much larger tank, something to look forward to.



    Adding six small fish isn’t normally a problem, but given your current low stocking I would check the ammonia and nitrIte readings for a few days afterwards in case of a mini-cycle.

    Water changes are an important part of aquarium maintenance. They remove accumulated organic acids and nitrAtes whilst refreshing the water by adding minerals. That said, it’s important to have a regular water change regime. Changing 50% once a month can be risky as the tank and tap water will have different parameters which can lead to osmotic shock as the fish are subjected to a sudden change in their conditions.

    It’s normally recommended to change 20-25% weekly, but providing frequent changes are carried out these can be larger if desired as the tap and tank water will be similar. I’ve heard of some breeders who change up to 200% daily!



    Hi Cmoss! I am wondering what the disease was that killed your first fish? I have seen protozoans go “dormant” and resistant survivor fish do just fine, only to have the disease reappear and knock out new fish. This happened to me once, and I had to change out the gravel and filter media, and boil the ornaments. After I did that, I never had the problem again.

    Regarding tetras and pictus…I have three pictus catfish in a 55gallon(US) tank. The smallest fish that have not been eaten by the full grown pictus (4”) are Brilliant Rasboras and Checker Barbs. Harlequins would be eaten. When you go tiny, go tiny all the way so you dont have to worry. Khuli loaches, horse face loaches, corydoras catfish, cool shrimps and snails…all good to go with small tetras. The rainbow shark wont eat anyone though…does he have a cave?

    Good luck, and I am sure the experts here will help you out.



    Thanks Eyrie, I will keep an eye on the ammonia and nitrate readings once the new fish are in. I will also change the way I clean the tank it makes sense when I think about it.

    Reva, That does sound as though it could have been the case, as it would explain a very puzzling event. After I lost the last batch of new fish I decided that a complete clean out was the only way I could eliminate whatever was killing the fish so I did a very similar thing as you had suggested worked for you, so fingers crossed the problem is solved.

    With regards to the rainbow shark, he does have a couple of areas he can hide away but maybe a cave wouldn’t be a bad investment, im sure I could make the space by moving some of the other ornaments around.

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