Q. Was this water contaminated?
I recently bought some Otocinclus catfish from a well known pet store. I added them to the tank but also added the water they were bagged in. Foolish, I know but I did it — and ever since I have had a wave of fish deaths. I’ve tried to remedy this with water changes and my parameters seem OK. What is the best way to introduce new fish? Is it possible that a pet shop’s water was contaminated?
AAdding new fish to an established tank always brings with it the possibility of introducing disease and parasites at the same time. The best shops fully quarantine their livestock before putting them up for sale but even this can’t guarantee that they aren’t harbouring problems. Wherever possible it is best for the hobbyist to quarantine new fish at home for several weeks where they can settle and any signs of disease or parasites can be spotted and dealt with.
However, many of us don’t have the facilities or space to do this, so we have to take the gamble of adding fish direct to the display tank. You don’t mention any symptoms that the sick fish showed before death so it’s very hard to work out what the problem is.
Not adding the water from the transit bag is usually a good idea, but diseases and parasites are more likely to be actually on or in the fish than in the water itself. Some shops keep constant levels of medication in their display tanks, especially marine fish where copper is often used to keep parasites at bay, and adding this water to a tank with inverts in it can cause real problems.
If your Otos were not visibly ill when you bought them, then there isn’t much more you could have done without quarantine facilities. Fish deaths around the introduction of new fish to a tank usually go two ways in my experience.
The first is that the new fish die soon after introduction, often due to a sudden change of chemistry from their stock tank to the home display, where the fish have often got used to a steady drop in water quality or chemistry without showing any obvious signs, meaning that the owner doesn’t test the tank, thinking that the water quality must be fine.
The second scenario is that the new fish are OK, but the existing stock drops like flies. This is usually due to disease, and happens in much the same way as something as apparently mundane as a common cold can wipe out an isolated community in the Amazon. The fish in your stock tank have lived in isolation for so long that they haven’t been exposed to any pathogens. The new fish are hardy survivors of all the nasties thrown at them during the process of capture, export and so on, and they bring with them diseases that they are largely immune to. Once added to the display the existing fish are rapidly wiped out while the newcomers carry on as if nothing is wrong in the tank.
My advice is to quarantine, but if this isn’t possible, then get a reading of the shop’s tank water (not the water in the bag which can change during transit) and compare this with your own aquarium water. Make sure they match as closely as possible.
Only buy healthy fish that are not sharing their sales tank with any obviously sick individuals.
When acclimatising the newcomers, do so in the usual way (see the step by step process on the right) and then net the fish out of the bag and dispose of the water in the bag afterwards.
Could Paul’s new Otos have brought something nasty into the tank with them?