Why is my office tank giving me problems? 


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When a bunch of dissimilar species all die at once, you can usually assume it’s environmental rather than disease. As Neale Monks explains… 

Q: I run two tanks at my home which are on mains water and they are stable; in 10 months I haven’t had any issues with fish health or survival. 

I also have a tank in my workplace office. However, this one is run from a private water supply, which provides the drinking water for dozens of cottages and a luxury spa and holiday site, filtered with 3 x 1 micron filters and disinfected with a very robust UV lighting system. The water has a pH of 7–7.2 and no nitrates or nitrites. It is clear of all mains water additions such as chlorine, with microbiological testing for the water supply coming back with excellent results. But the tank has become a real challenge. It’s a 100 l set-up, with plenty of healthy, lush plants. For four months over summer the tank was blissful, but since the wetter autumn weather has arrived in the Lake District and run-off from the surrounding land has increased considerably, I’ve lost 90% of the fish (Cherry barbs, tetras, corys) and all of my Cherry shrimp. The water supply readings remain stable, the water is still as fresh and of high quality, my maintenance/water change routine remains the same, but I have lost nearly all the livestock in under a month, with no signs of disease or distress. Have you ever heard of anything like this on a private water supply? Please help! 


A: Neale replies: When a bunch of dissimilar species all die at once, you can usually assume the issue is environmental rather than a disease. Pathogens tend to pick fish off one at a time, usually with some obvious symptoms, and in many cases affecting one species more than the others. So, let’s look at what the environmental issues might be. 

The obvious pick with any sort of treated water is what’s added to the water to make it safe to drink. Municipal water supplies add chlorine or chloramine, and these are both very toxic to fish. Aquarium water conditioners neutralise these, and it’s worth remembering that adding a good quality water conditioner to new water is always a good idea. You say that chlorine isn’t used, but what about chloramine?  

I’d also check that copper isn’t the problem. Again, most modern water conditioners will detoxify heavy metals like copper, but soft water especially reacts with copper and even low levels of copper can be acutely toxic to fish, and especially invertebrates such as shrimp. Any aquarium shop that caters to marine fishkeepers will have copper tests in stock, and may well do a test for you at minimal cost if you don’t want to buy a complete test kit. 

The next thing to consider is the total dissolved solids. General hardness and carbonate hardness are the two aspects we tend to focus on in the freshwater side of the hobby. Very soft water isn’t ideal for a lot of fish, at least in captivity. To be fair, if the carbonate hardness was very low, which allows rapid acidification between water changes, you’d expect to see variation in pH rather than the apparently stable pH readings you’re reporting. So, that’s odd. Very low general hardness is more of an issue in terms of osmosis, and while some species handle this well (your classic blackwater species for example) other, more commonplace community tropicals do prefer at least moderate hardness. I’d be aiming for a general hardness of around 5˚dH even for a collection of soft water species such as tetras and Corydoras. 

The third thing to consider is whether sodium salts are added to the water at some point. This most commonly happens with domestic water softeners, where the aim is to create water that lathers well and doesn’t cause limescale. Such water may be good for washing and cleaning, but it isn’t good for aquaria! There’s some debate about whether such water is even good for we humans to drink, but in many cases a bypass tap is fitted in the kitchen for people to drink from instead. 

Finally, there’s ‘bad luck’ if you will, where something external to the water supply, such as painting or cleaning products, have been used that have contaminated the aquarium. This can be a difficult problem to identify with certainty, but the use of activated carbon in the filter will help remove anything dissolved in the water, while a series of water changes can dilute such contaminants. Of course, if the aim is to maintain a steady set of water chemistry parameters, doing large scale water changes may be undesirable. 

My gut feeling is that the use of a commercial Discus buffer (to stabilise and optimise water chemistry) alongside a good quality water conditioner will provide the best approach to preventing further problems. Activated carbon might be well worth using, too — not something I say very often!  

I’d also reiterate to those around the aquarium, such as cleaning or maintenance staff, that care should be taken when using things like cleaning products and paint.  

I’m sorry I can’t give you a more definite answer, but in situations like this, it’s very hard to say anything more concrete.