I have a problem that nobody has heard of. In my gravel, there are little black dots. With a magnifying glass, they look like little fleas or ticks. You can see the legs and watch them move. The problem is they kill all new fish. All the older fish succumbed — even the catfish. A dead fish is gone in hours. Thanks for any help.
Let’s discuss some possibilities. First off, I doubt these tiny critters are killing your fish. It is more likely that they are scavengers. Hobbyists see organisms on a dead or dying fish and assume they murdered it when it’s much more likely that they are merely opportunistic scavengers. This is backed up by your statement that a dead fish is gone in hours. Parasites don’t feed on dead hosts — scavengers do.
This may point to insufficient maintenance as a probable cause of your problem. First, insufficient water changes would let the water chemistry deteriorate to a point where it kills the fish. It would also let populations of scavengers develop to a point where they could consume a dead fish in hours. Regular partial water changes, using a gravel vacuum, would clear up both issues. It would freshen the water and siphon out most of the microbeasties, preventing them from developing large populations. Check your ammonia, nitrite and pH levels to be sure that they are where they should be.
Now, let’s look at possible identifications of the critters themselves. Your description described them as “black dots,” “fleas” or “ticks.” To me, those three things look very different, but here are some possible organisms that fit each description.
To me, “dot” suggests round. Ostracods are round and could have dark color. A common name for these critters is “seed shrimp.” They have a shrimplike body encased in a clamlike shell and resemble Daphnia. Their antennae poke out the front of their shell, and the seed shrimp use them in a rowing motion to swim. They also use them to feel and filter bits of food from the water. The antennae could resemble legs to the observer. Seed shrimp grow to around one-eighth inch.
Interestingly, ostracods are parthenogenic during the summer, meaning that females give birth to clones; so, if one seed shrimp hitchhikes into your aquarium on an aquatic plant, it’s possible to have a whole swarm in no time. It’s not rare for aquarists to suddenly notice a swarm of them swimming about the aquarium. Don’t worry — they are harmless scavengers, and many fish snack on them.
Your description of “flea” perfectly fits the description of Gammarus shrimp, also called scuds or sideswimmers. They are shaped much the same, with a thin but tall body. They grow up to a half inch, though, so they may be too large for your description as “dots.” Scuds arrive as hitchhikers on aquatic plants, and live in the gravel and detritus. Hobbyists often spot them zooming and looping around the aquarium. They, too, are harmless scavengers, and I’d consider them to be desirable. Most fish relish them as a snack. Gammarus shrimp would happily congregate on a freshly dead fish to enjoy a hearty meal.
I can think of only one group of aquatic animals whose shape could be described by the word “tick”: the water mites. Those, too, sometimes arrive as hitchhikers on aquatic plants. Some are brilliant red and quite attractive, and there are green, brown and black species, too. As larvae, water mites are mostly external parasites to aquatic insects and small crustaceans. They bite into the soft tissue between the hosts’ joints.
Though some adult water mites remain parasites, most become independently living predators to tiny aquatic insects and crustaceans. They actively hunt them, pierce them with their mouth parts and inject a digestive saliva that dissolves some of the tissue, which is sucked out for food. They’re a bit like spiders that don’t create webs. Some species are herbivores as adults.
Though I have never heard of an actual case, there are species of water mites that can act as gill parasites to fish. Perhaps your aquarium has acquired one of them. If so, a visit to your local aquarium store to obtain a medication containing copper sulfate or trichlorofon would be the remedy. (The same treatment would also destroy the harmless scuds and seed shrimp, though.) The problem with this diagnosis remains that if gill mites killed the fish, they would drop off to find a new host or die along with it. They wouldn’t consume the dead body.
Finally, since you say these mystery animals have legs, they could be hatchlings of any of hundreds of species of aquatic insect larvae. However, I can’t think of any species that would be predators to fish at such a tiny size. Happy fishkeeping!