Basics : Disease Control
Generally, hobbyists should not try to become ichthyologists (those who study fish) or veterinarians. Our focus should be on keeping good water parameters (which are stable and agree with the fish we are keeping), supply a proper diet and follow basic guidelines for isolating or preventing the spread of organisms.
The single best disease control (after proper water conditions are establishing and being maintained) is to not add anything aquatic to your aquarium. New fish, plants, invertebrates are all potential carriers of some new contagion which will frustrate and confound you. However, not being able to grow your hobby would be unacceptable, so I've got a few strategies for your consideration.
It's important to note than in nature, fish are all connected to each other by water, which is an excellent medium for transporting parasites and diseases. All fish would have by now gone extinct if it was not for their very powerful immune systems. Other than complete isolation, the next best disease control method is keeping your fish very healthy. In an aquarium, there are 2 disadvantages imposed on our fish. The first is that their immune system gets lazy (nothing to keep it very active). The 2nd is that when a contagion does appear, the close quarters of an aquarium make it very difficult for the fish's immune system to protect itself in time, because the concentration of disease increases very rapidly (so water changes are another excellent disease control method, buying the fish time to naturally defend itself).
Details...
Topics briefly discussed are general diseases, susceptibility, treatments, staged tank setups, isolation tanks and quarantine tanks.
DISEASES
Basically, there are parasites and bacterial infections. Of a lesser concern is
fungus, and then there are the rarely occurring viruses. Of the bacterial infections,
most are classified as gram negative, and we generally use antibiotics to combat
them. Parasites are usually destroyed with dyes such as malachite green, though
you need to wait for the parasite to reach the life cycle stage where it will
be exposed to the dye. Copper can also be used against parasites, as it's toxicity
is well documented, but the risk is in that too high a dosage will also kill the
fish. Fungus is naturally occurring and will make an appearance when a stressed
fish has an open wound. Healthy fish are not normally bothered by fungus, even
with open wounds. The tips of a fish's fins are particularly vulnerable to bacterial
infections, so when they are stressed (from other non-optimal conditions), tail
& fin rot may occur, from either fungal or bacterial origins. In this case, medications
to arrest the fin rot are usually needed until the fish can develop its own antibodies,
or the particular strain of bacteria is completely eradicated from the tank. Quick
identification is essential for successful treatment. These links
are extremely helpful.
SUSCEPTIBILITY
In a community tank, fish will often be affected according to species, as each has particular immunities and susceptibilities. If a disease gets established on a susceptible species, it will use that host to saturate the water and attack the less susceptible fish. In this case, the prescribed treatment is to both medicate the tank, and if possible, isolate and medicate the host(s) in a separate hospital tank. Never add fish to a tank under treatment (unless it is for the same medicinal purposes). Non-susceptible fish may become carriers causing contagions to linger for long periods of time. The toxicity of many diseases increases when the host dies, either from open wounds spreading the contagion, or through ingestion as internal bacteria and parasites spread themselves. Diseased fish close to death should be removed to isolation.
TREATMENTS
Follow the directions on the package. Note whether a change of dosage is required for scaleless fish (loaches) or for small-bodied fish (tetras). Many medications require the activated carbon to be removed. Many medications react badly with your good nitrifying bacteria, so an ammonia or nitrite accumulation will need to be corrected (ammonia locking products, aquarium salt for nitrite, or water changes for either one). The UV in natural light will break down many medications.
The treatment of Ich should span the life cycle of this parasite, which varies according to the temperature of the water. Many variations on manufacturer's recommendations exist, including the use of salt, heat and not using any medication. Typical manufacturer's recommendations for the treatment of Ich are of too short a time period to do more than to knock back the parasite, and weaken successive batches, however, this is often sufficient to do the job (your results may vary).
STAGED TANK SET-UPS
Starting with a new tank, set it up with the total quantity of fish, plants etc which you want to have for about 1 to 2 years time. The fish are all juveniles, and you have purchased a few extra of each species to allow for some mortality. Leave the tank alone for about 3 months, carefully monitoring ammonia and nitrites. This effectively becomes one large quarantine tank which is given 3 months to completely stabilize. If you want to add more fish, plants etc, put them in another tank. If you have 2 tanks running problem-free, you can move product between them. Any new introductions into a tank should cause it to be treated as if it's an isolation tank for at least 2 weeks (3 is better).
ISOLATION TANKS
Tanks in isolation have their own nets, algae scrubbers, algae magnets etc. Avoid doing tank maintenance on all your tanks on the same day. Avoid the ISO tank when working on the others (and avoid your other tanks when working in your ISO tank). A wet arm will transport diseases very effectively. There is no fish/plant movement out of an ISO tank, and preferably no movement into it either.
QUARANTINE TANK
New purchases are placed into a Q-tank for observation, typically 2 to 3 weeks. Follow ISO-tank procedures. Q-tanks typically have no substrate, simple sponge filter (easy to disinfect), submerged heater (no canopy gaps for nervous fish to jump through), are not lit (calm somewhat dark relaxing environment), have suitable inspection light nearby (automotive repair lamp on a cord works well), are away from house foot traffic and the fish are fed a monitored premium diet.
During quarantine, you are watching for any atypical behaviour from the fish.
Fin clamping, rapid respiration, colour changes, no or poor feces or appetite,
etc are trouble indicators.
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