(Unedited version)
Originally appeared in the July 2008 issue of
Tropical Fish Hobbyist Magazine

When most aquarists think of freshwater invertebrates those that come to mind most often are probably shrimp and snails, or perhaps crabs or crawfish. There are many other invertebrates that live in freshwaters that can be introduced to the home aquarium whether planned or not.

Many animals can come in on plants and this makes it particularly important for those of us keeping planted tanks to be aware of what animals might pop up unexpectedly in our tanks and what to do about them.


Hydra look like very tiny sea anemones, and they’re related. Jellyfish Corals and sea anemones are among the animals in the phylum Cnidaria (Coelenterata). Hydra are unusual biologically because though they don’t have eyes they can sense light. Hydra can be white, brown, or green and can attach to almost anything in the aquarium, plants, decorations, gravel, the aquarium itself. Hydra can reproduce easily in the aquarium

Hydra are carnivorous and eat pretty much any animal they can manage to catch. They can kill small fry, and will be happy to eat any live foods you are trying to feed your fish. To kill their prey hydra shoot individual cells called nematocysts from their body and tentacles. The nematocysts paralyze and kill the prey, which are then digested by the hydra.

Hydra can do particularly well in the aquarium if there is an abundant source of small live foods. This can be a devastating combination for some fish breeders; with lots of food for the fry feeding the hydra, and then the fry themselves becoming hydra food. I do occasionally see hydra in my tanks but they have never become populous enough to be a problem for me. Gouramis are reported to eat hydra. There are also several commercially available medications reported to be affective on them. In most decorative planted or community aquariums, just keeping the tank clean tank and not overfeeding should keep hydra from multiplying to the point of them becoming so much of a problem that medications are needed.

Moss Animals

More correctly called Bryozoans; moss animals are widespread, colonizing, animals which are also abundant in the fossil record. Most bryozoans are marine, but some have moved into fresh and brackish waters. They can also find their way in to your aquarium.

Bryozoans are made of colonies of individual zooid. The bryozoans most often seen in the aquarium, form small branching colonies. Some species form very different types of colonies. Very large colonies that look like giant egg masses are found in some lakes.

The presence of moss animals in the aquarium is often short lived, usually no more than a few months, and they shouldn’t be a problem. In some aquariums they may occur somewhat regularly. Colonies not only will grow but sometimes zooids will produce statoblasts, an asexual form of reproduction. Statoblasts, are somewhat similar to seeds in plants. They are resistant to adverse conditions and can germinate to form a new colony when conditions improve.


Planaria are often mistaken for leeches because they both have a flat worm like appearance. In fact planaria are in a class called flatworms or Turbellaria. Planaria I’ve seen in the aquarium have been in shades of white, brown or green, though other colors can occur, some with patterns. Planaria often have a somewhat arrowhead shaped head like area, with eye spots. The eye spots are light receptive but not true eyes.

Planaria glide across surfaces in the aquarium. They generally eat much smaller animals that live on those surfaces. Some species of planaria will even ingest hydra and store the hydra’s nematocysts for its own future defense. They may also feed on fish eggs.

Planaria are not uncommon in the aquarium but large populations of them usually mean that aquarium is having some pollution issues. Overfeeding, over crowding, inadequate filtration and water changes can all contribute to these types of infestations.


The idea of leeches often gives most people a bit of the heebie jeebies. Though a few leeches feed on mammals most prefer a cold blooded lunch, and a few are scavengers or even eat plants. Leeches are usually introduced through plants or live foods that came from natural sources or ponds. Sometimes I’ve seen wild caught fish that have had leech infestations also.

Leeches differ from planaria in that they are segmented and have suckers at both ends. This gives them the ability to move through surfaces in the water like an inch worm. Leeches are variable in color but the ones I’ve seen on fish or in the aquarium have been plain light tan, or brown. Ones I’ve had the displeasure of having been eaten by while collecting have been brown.

I haven’t had much experience with leeches on my own fish, though I have seen them in store tanks, on wild caught fish, and hitchhiking in on things I’ve collected, or come from someone’s pond, that have been placed in quarantine tanks. For leeches that are infecting fish, the most recommended remedy is a salt water bath for the fish. There are also medications available in most pet stores that may help for more severe problems. Since different species of fish have different tolerances for both salt and medications you should research your specific fish before treatment. On line searches and books are good places to start your search.

I should also mention that some people have aquariums specifically for keeping leeches in. Some leeches can get quite large and many have interesting color or patterns.

Mosquito Larvae

Mosquito rarely show up in waters that have fish in them but will very often show up, as if by magic, in almost any water left standing in warm enough weather. This certainly includes any aquarium that has no fish in it, or even the water left in a saucer of a house plant, or any container that might have water in it outside. Mosquito larvae can show up almost anywhere there is still or slow moving water.

Both male and female mosquitoes eat nectar, but females need blood to lay their eggs. Different species of mosquitoes like to eat different species of animals, including, of course, humans. So after eating off of you, a family member or guest, the female mosquito finds water to lay her eggs. They can lay up to 200 eggs in little floating groups called rafts. The eggs generally hatch in the next few days dependent on conditions, and then go through 4 larval stages. They hang head down at the water surface breathing through a siphon tube. In about one to two weeks the larvae turn into pupae. They can still move when disturbed, but not as well, as the larvae. The pupae have no mouths so can not eat, and look like large commas hanging in the water. They emerge from this stage, in just a few days, as full fledged adult mosquitoes.

The best way to get rid of mosquito larvae in an aquarium is to add a fish. There are very few aquarium fish that won’t be very happy to eat mosquito larvae. If they can catch them, and usually they can, shrimp seem to love them too.

Damsel and Dragonfly Larvae

Dragonflies and Damselflies are from the insect order Odonata and were flying the earth before the dinosaurs ruled. We recognize these interesting and beautiful creatures from their final adult flying stage, but most of their lives are spent in the water in a larval stage.

These larvae can come in on plants, or anything collected in water outdoors. The larval stage usually last from one to two years. Larvae and adults are predacious and are happy to eat not only other invertebrates but also fish. They are very good at camouflage. If one sneaks in to your aquarium and its heavily planted you may never even notice it’s there until some day it starts flying around your home.

Generally dragonflies look larger and than damselflies. More specifically the dragonflies’ back pair of wings are larger than the front pair, while both pairs are the same size in damselflies. Damselflies can also fold their wings behind their backs, while dragonflies lack this ability. In larval form the dragonflies have a larger body. Damselflies are slender and have three feather like tails, which are its external gills.

Other invaders or occupants

There are thousands of other freshwater invertebrates that can be introduced into the aquarium; most of them are quite small, some can be a nuisance, but there are also many that are attractive, interesting and able to thrive in an aquarium. There seems to be more interesting in freshwater invertebrates lately. Certainly there has been a lot of interest in snails and shrimp and I see more people posting web sites and experiences with other freshwater invertebrates.

Looking back at all the advances made in aquarium keeping over the course of my lifetime, and then considering how much faster advances have been made since the popularity of email and web pages has grown, makes me believe that more freshwater invertebrates will become available in the next several years.

I can certainly see the star of a 10 gallon aquarium as large predacious diving beetle hunting down live prey; or an underwater scorpion, a walking stick that sucks the bodily juices out of its victim like a vampire, leaving a limp carcass to toss aside.

In the course of your planted aquarium keeping hobby you may run into some unexpected invaders that hitch a ride in with your plants. Most of these animals are harmless, some of them even make great fish food, populations of others could indicate possible problems in your aquarium, and some are veracious hunters that can eat fry or fish. Which, depending on your point of view, may or may not be a good thing.

Welcome to the Jungle | Into The Forest | The Creepy Crawlies | A Clearing in the Thicket | Algae Eaters for the Planted Aquarium
North American Natives | Why things go wrong Pt 1 | Why things go wrong Pt 2 (Algae) | Algae Eating Shrimp | Lo-Tech Tanks
Welcome to the Fish Room | The Stemmed Plants | Mosses | A Livebearer Biotope | Planted Tank Social | The Genus Hygrophila | Cyanobacteria
Easy Plants | What I Did Last Summer | Decorations in the Planted Tank | Botany-An Introduction to Plant Biology | Botany-Anatomy of a plant
Botany-How Plants Work | Easy Rosettes | Going High-Tech | Floating Plants | Dealing with Success | Bringing the Outside In | Vallisneria
Hair Algae | Flowering Aquarium Plants Part 1 | Flowering Aquarium Plants Part 2 | Liverworts in the Aquarium | Elements of Design
Planted Aquarium Maintenance | More Mosses | Invaders | Ferns in the Aquarium | Setting up a Planted Aquarium
Seaweeds of the Pacific Northwest | Proserpinaca | Hardware for the Planted Aquarium | Rotala | Neocaridina Shrimp | Lo-tech Tank Tips

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