Does adding fish to my aquarium add more diseases?

As we routinely receive livestock from suppliers all over the world, we know that fish from different sources can come with different pathogens. It’s the reason why all of our fish go through routine treatment cycles before they’re ready to leave our stores. You might occasionally see a batch of fish with white spot or other health issues (but usually none at all) behind our ‘Not for Sale’ labels waiting to complete this acclimatisation period.

By the time they’re heading for customers’ tanks, they should be free of disease. As moving to a new environment can be stressful, this is often a time when fish are susceptible to any bugs they may encounter and exposure to high nitrates, bullying from established fish, or a trip to a supermarket car park on a hot (or cold) day on the way home can all be sources of stress which result in new fish succumbing to parasites that established ones are accustomed to. Rather than bringing in new disease, the new fish has its immune system compromised by stress and picks up an illness from an established aquarium or pond. As the settled inhabitants remain healthy, the new fish then get blamed for introducing a disease which was there all along but held in check by the previous population. We see this a lot in tanks where nitrate levels have been allowed to rise over time and existing fishes have slowly adapted to the conditions which then harm newly added fish, even of the same species.

The fact that there always seems to be a pathogen waiting for the chance to nobble stressed fish seems to add weight to the theory that fishes and their disease organisms live in a state of balance, where their immune systems keep them in check. Looking at wild populations, it’s clear that parasites are often found in the gut of healthy animals and only when the fish is stressed do they become a problem. Many disease organisms seem capable of surviving away from a fish host, sheltering in detritus or as an encysted stage with few demands. In this way, they may live undetected until circumstances change, and a stressed host gives them the opportunity to multiply to levels that can overwhelm previously healthy fishes. By taking routine microscope samples of batches of fish, we’ve seen that parasites are capable of targeting a weaker individual amongst a healthy population – quite an achievement for a microscopic creature!

So, in summary, adding fish can result in disease outbreaks but this is often highlighting an existing underlying problem. Before blaming the new additions, investigate and remedy possible causes where necessary to avoid further losses.