You know that sinking feeling when you’re doing your routine tank check and spot something that definitely wasn’t there yesterday? Yeah, I’ve been there more times than I care to admit. Last Tuesday I was watching my tetras during my morning coffee (because apparently this is what my life has become) and noticed these tiny white threads wiggling on the glass. My first thought was “well, that’s new and horrifying.”
Welcome to the wonderful world of aquarium pests and parasites – the uninvited guests that show up to your carefully planned underwater ecosystem and decide to make themselves at home. After eight years of maintaining classroom tanks and dealing with whatever nightmares seventh graders accidentally introduce to my carefully balanced aquatic environments, I’ve become something of an expert in identifying and evicting these unwelcome visitors.
The thing about aquarium pests is they’re like that relative who shows up unannounced and overstays their welcome, except they multiply exponentially and potentially kill your fish. Not exactly the same level of awkwardness, but you get the idea. And trust me, in a classroom setting where twenty-five kids are watching your every move, you better believe I’ve gotten good at dealing with these situations quickly and effectively.
Let’s start with the usual suspects, because once you know what you’re looking for, half the battle is won. Those white threads I mentioned? Planaria. Little flatworms that appear out of nowhere, especially when someone (looking at you, Kevin from third period) decides your fish need extra food “because they look hungry.” These guys are actually kind of fascinating from a biological standpoint – they can literally regenerate from tiny pieces – but they’re absolute menaces in a fish tank.
Then there are hydra, which sound way cooler than they actually are. These tiny creatures have tentacles that can sting and capture small prey, which means your baby fish or shrimp are basically swimming around with tiny sea monsters. I discovered hydra in my classroom tank during a particularly stressful week when I was dealing with parent conferences and completely forgot to do my usual detailed tank inspection. Came in Monday morning to find my cherry shrimp acting weird and skittish, hiding in corners instead of grazing happily on algae.
Don’t even get me started on snails. Now, I actually like snails – they serve important functions in aquarium ecosystems – but when you’ve got pond snails or bladder snails that reproduce faster than you can count them, suddenly your tank looks like a snail apartment complex. I learned this lesson the hard way during my second year of aquascaping when I didn’t properly quarantine some plants from a local fish store. Within a month, I had what my wife dubbed “the great snail invasion of 2019.”
The key to managing any pest problem is early detection, which means becoming a tank detective. I’ve gotten into the habit of really looking at my tanks, not just glancing at them. You need to examine the glass, the substrate, the plant leaves, even the filter intake. I keep a small magnifying glass in my classroom desk drawer specifically for tank inspections, which probably makes me look like the world’s most boring detective, but it works.
Some of these pests are masters of camouflage and timing. Planaria, for instance, often hide during the day and come out when the lights are off. I’ve started doing occasional nighttime checks with a flashlight, which revealed way more activity than I expected. It’s like discovering your tank has this whole secret nightlife you never knew about.
When it comes to actually dealing with these invaders, I’ve learned that patience and strategy beat panic every time. My first instinct used to be reaching for whatever chemical solution promised quick results, but that approach backfired spectacularly during my third year when I nuked beneficial bacteria along with the pests and crashed my nitrogen cycle. Spent the next two weeks doing daily water changes while my students asked why all the plants were turning brown.
Natural solutions work best when you can make them work. For snail control, I’ve had great success with assassin snails, which are basically the hitmen of the snail world. They hunt down pest snails methodically and efficiently, and unlike chemical treatments, they won’t harm your beneficial bacteria or stress your fish. Plus, watching an assassin snail track down its prey is actually pretty educational – I’ve used it for lessons about predator-prey relationships.
Environmental management is probably the most important long-term strategy, though it requires some patience. Most pest problems stem from imbalanced conditions – overfeeding, poor water quality, excessive nutrients. I learned to think of pest outbreaks as symptoms rather than the actual problem. When I had my epic algae battle (and I mean epic – the tank looked like a green soup for weeks), the real issue wasn’t the algae itself but the fact that I’d been running my lights too long and my nitrates were through the roof.
The solution involved dialing back the photoperiod, increasing water change frequency, and adding more fast-growing plants to outcompete the algae for nutrients. It took about a month to see real improvement, which felt like forever when you’re staring at what looks like a science experiment gone wrong, but the results lasted.
Sometimes you do need to break out the serious tools. I’ve used targeted treatments for particularly stubborn problems, but only after trying everything else. There are aquarium-safe medications that can eliminate specific pests without harming fish or beneficial bacteria, but you have to be precise about dosing and follow instructions exactly. I learned this when I underdosed a planaria treatment and basically just annoyed them instead of eliminating them.
UV sterilizers have been game-changers for certain problems, particularly green water algae blooms. I added one to my home setup after dealing with a bloom that made my tank look like pea soup for three weeks. The difference was dramatic – within days the water started clearing, and within a week it was crystal clear again. It’s not a solution for every pest problem, but for free-floating parasites and algae spores, it’s incredibly effective.
The classroom environment has taught me a lot about prevention, mainly because when something goes wrong with twenty-five kids watching, you better have a backup plan. Quarantining new plants and fish isn’t always practical in a school setting, but I’ve learned to at least give new additions a thorough inspection and a preventive treatment when possible. Most pest introductions come from new plants or fish carrying hitchhikers.
I’ve also learned the value of having allies. The local aquarium club has been incredibly helpful – experienced hobbyists who’ve dealt with every possible pest problem and are happy to share solutions. Online communities too, though you have to filter through a lot of conflicting advice. When I was dealing with a particularly persistent case of what I thought was ich but turned out to be velvet disease, input from other aquarists helped me identify the correct parasite and choose the right treatment.
Documentation helps more than you’d expect. I started keeping a simple log of what I observe, what treatments I try, and what results I get. It’s helped me recognize patterns and avoid repeating mistakes. Plus, when you’re dealing with the same pest problems across multiple tanks, having records of what worked where saves time and frustration.
The reality is that pest problems are just part of aquarium keeping. Every single person I know who’s been in this hobby for more than a year has stories about battles with various unwanted critters. The key is not to panic, take time to properly identify what you’re dealing with, and choose treatments that won’t cause more problems than they solve. Your tank will recover, your fish will be fine, and you’ll have learned something new about aquatic ecosystem management. Usually.
Tom teaches middle-school science in Portland and uses aquascaping to bring biology to life for his students. His classroom tanks double as living labs—and his writing blends curiosity, humor, and a teacher’s knack for explaining complex stuff simply.
