Does medicating an entire tank harm healthy fish?

Question

I have a female guppy, that's been through a lot of stress over the past year. She's moved from my friend's overcrowded, unclear green algae tank to mine, has had babies 4 times, has had internal parasites (was separated into a hospital tank, treated and put back in original tank after the parasitic infection was treated), and now appears to have dropsy. I do not have a hospital tank available, it's currently turned into my corydora breeding tank.

The first recommendation for any fish with dropsy is to quarantine the fish, and medicate separately. My question is, if a quarantine tank is not available, is there any harm in medicating the entire tank? My tank currently houses guppies, balloon mollies, a snail and Chinese algae eater.

Answer 1

As you've already pointed out, the best course of action is to quarantine the fish. If you really can't do this then you'll need to tread carefully with medicines if you have any invertebrates (snails, shrimp) or fish without scales (bristlenose catfish, loaches, etc.).

Ultimately, this comes down to reading the label of the medicine. There is such a smorgasbord of different medicines that it's difficult to put a hard and fast rule on which to use (they almost always warn you about the fish that might react adversely). While reading the label, you'll definitely want to check that the medicine "doesn't harm your filter" and whilst many claim they won't, there is a chance that it could negatively impact your beneficial bacteria anyway.

Remember that when you add medicine to a tank, you change what the water contains. This can be stressful for fish which are healthy or fighting off an issue already. I've had great success with Melafix and Pimafix together as a 'general cure' and my catfish, snails and loaches have not had any issue. Whether this will work for dropsy on an older fish anyway, that's difficult to say.

As ever, my overriding advice would be to keep your water quality as clean and consistent as you possibly can during periods of ill-health. The consistency is important because big changes in your water can tip sick fish over the edge.

Finally, older or stressed fish can become more susceptible to ill health which might explain why dropsy (normally occurring in poor water conditions) has happened in a tank with good quality.

_{After thought: Remember the cost of this! Medicating a whole tank can be much more expensive than medicating a smaller tank.}

Answer 2

I've heard that using unnecessary medication on healthy fish may harm the kidneys of other fish. Also as the sick fish may have dropsy it is highly suggested that you quarantine as it can be contagious and avoid unnecessary medication as dropsy can be caused by kidney issues and as stated before unnecessary medication can harm the kidneys of the fish. I suggest (as it can be inexpensive but I kinda hate myself for suggesting it because it feeds misconceptions about betta care) is to buy one of those tanks advertised as good for betta, like 1 gallon ones, and use that as a small hospital tank. (Edit: sorry I didn't see how old this thread was)

Answer 3

There are too many types of medication to give a general recommendation.

There are certain medications, like those that use the ingredients malachite green or formalin that are very hard on invertebrates (snails, shrimp) and are harder on scaleless fish (catfish, loaches) and softwater fish (cardinal tetras, German Blue Rams, discus). These are also tough on plants. I would advise against treating in a community tank unless it is thought the root problem affects all the fish and the fish are likely to tolerate it at the necessary dose. These two ingredients can kill your nitrifying bacteria, but they may not.

Antibiotics (kanamycin, erythromycin, tetracycline, others) are generally better tolerated by a wider variety of critters and under the right circumstances are relatively safe to apply to an entire aquarium. With that said, some of them will kill your nitrifying bacteria, thereby un-cycling your aquarium and making it likely that you will deal with persistent ammonia spikes that will kill or badly harm your fish. Research the ingredient carefully to assess the risk here.

Copper is an ingredient that can be very effective for certain ailments but again is tough on scaleless fish and in general kills fish if overdosed. It is absolutely lethal to invertebrates and is difficult to remove from the tank. Even just trace amounts left over from treatments long ago can kill snails and shrimps. I would never apply copper to a community aquarium.

There are some anti-parasitic medications, like metronidazole and praziquantel, that are very mild and shouldn't pose serious risks in a community tank. I still wouldn't put them in a tank unless I felt certain that some of the fish needed it; I wouldn't put them in a tank as a purely precautionary measure.

As a general rule, find out what the medication's active ingredients are, read through the warnings and recommendations of the manufacturer, and then research the ingredients individually to learn from other aquarists and sources.