July 13, 2025
Starting up an aquarium sounds easy, right? Get a tank, fill it up, toss in some fish, boom—hours of sitting in front of the aquarium watching your fish achievement unlocked. Yeah... not quite. I wish I could say I nailed it first try, but I definitely sent some poor snails and tetras to the great aquarium in the sky before figuring things out.
So, in the interest of saving your fish (and your sanity), here’s a list of the biggest mistakes new fishkeepers make. Read on, laugh at my pain, avoid repeating it. Or ignore it and learn the hard way. That’s also an option.
1. Not Cycling Your Tank First
You know how you wouldn’t want to move into a brand new house if the plumbing wasn’t hooked up yet? Same thing for fish, except the plumbing is bacteria that clean up their waste. Learn about the nitrogen cycle, and no, it’s not as complicated as your high school biology class made it sound. My biology teacher had a squirrel, but I digress...
I’ll admit it: I skipped this step my first time. RIP, Oto catfish. We listened to the advice from the gal at the big box store who told us that the black mystery snails are more aggressive than the blue ones. Had we done research first, that bottle of quick start (and the 7 purchased after it) wouldn't of been necessary... well, unless we did a fish in cycle. Remember the squirrel? My bad, back on track.
How to avoid:
- Look up “fishless cycling” or nitrogen cycle on Google/YouTube/Discord/Reddit/Facebook/Instagram... whatever floats your aquarium sized boat. Have you seen those by the way? Super cute. Little dude fishing on the top of your aquarium! Oh, hello squirrel I saw something shiny.
- Don’t put fish in until your water tests show 0 ammonia, 0 nitrite, and some nitrate.
- Patience, young Padawan, as my husband loves to quote. It takes time. The fish hobby is definitely not about instant gratification. If you are looking for that, I'm sure there is a 15 second video of an AI fish on Insta that you can watch.
2. Overcrowding the Tank
Look, I get it. You see all those colorful little guys at the local fish store and think, "I want them all!". This ain't Pokemon... well, unless you are referring to rainbow fish, that one I am trying to collect them all. Jam-packing your tank is basically turning it into fish Hunger Games and trust me, the odd's will not be in your favor.
That cute little pleco you bought that was labelled as common pleco? It's not a bristlenose that will max out at 5", it's a common pleco and it won't be staying that size. It’s going to grow into a tank-wrecking monster faster than you can say, “Why does my water smell like a swamp?” And what about Goldie? Yah, she's gonna be frickin' huge too and make a complete mess in your tank. Hope you like plastic plants.
Avoid it by:
- Googling how big your fish get as adults BEFORE you buy them. Ask the person at the fish store. If you are in a store that doesn't rhyme with metshmart or fetchco, you should be able to trust the advice they are giving. Want to be further confused? Everyone has their own opinion on what "should be done" vs. what "is acceptable". Get info from multiple sources then make a judgement call based off of the information that you have been given.
- Following that 1 inch of fish per gallon rule—loosely, because there’s always exceptions. I don't follow it, but I do more water changes to compensate. See number 3 below.
- Leaving room for fish to grow up. They’re not action figures and for Pete's sake do not get them a "for now" tank. Spend the money, get the tank that will be their forever home. Unnecessary moving of the fish is stressful, as is bad water quality, not enough swim space for what they like, and some species like friends. Do your research.
3. Ignoring Water Quality
If you wouldn’t drink from your fish tank, your fish shouldn’t be living in it either. High ammonia, nitrites, and leftover food sludge turn that pretty glass box into the aquatic version of a gas station bathroom.
Don’t be that person:
- Do your water changes — check your param’s to see how the water is doing, then decide how much (if any) you should change out. If all is going swimmingly (see what I did there?), then you shouldn’t have to take out much. But if your nitrate test matches the red of your checkbook after a trip to your local fish store, then… change that S.
- Make sure you got a solid filter rated for your tank size.
- Test your water regularly with actual test kits. Strips exist, but liquid tests are way more accurate and easier to interpret. There are some good brands out there though and some of the more seasoned hobbyists swear by test strips. You do you, boo.
- Stop overfeeding. I know they look hungry. They're always hungry. My two cats are always yelling at us for food, we know they are liars because one of us will have fed them 30 minutes before. Fish do the same thing. Ignore the big doe eyes of that Angelfish... he just ate 5 worms, a couple of his future children that you didn't realize were laid on that sponge filter, and sucked all of the algae off of a leaf. He's good.
4. Adding Too Many Fish Too Fast
Slow your roll. Dumping 20 fish into a brand-new tank is like opening a new restaurant and seating 100 people before the kitchen’s even built. Things will go bad fast.
Fix it:
- Add a few fish at a time over a few weeks. Let the bacteria keep up.
- Research which fish get along—cichlids vs that small tetras isn’t exactly BFFs, or even frenemies - more like a trip to the local sushi restaurant.
5. Buying Fish Just Because They Look Cool
We’ve all done it. You see some flashy fish, grab 'em, and only later realize… oh no. Turns out, that gorgeous angelfish you couldn’t resist? Yeah, he sees your adorable school of chili rasboras as appetizers. RIP rasboras, you will be missed.
Be smarter than me:
- Research before you buy. Water temp, pH, adult size, tank size, attitude—know it all. Utilize Google, friends, family, other fish keepers, social media, YouTube videos, the library (do they do the Duey Decimal system still?)
- Plan for long-term care. That cute fish might be chill now but give it a few inches of growth and suddenly your peaceful community tank looks more like a Terentino movie scene.
6. Not Paying Attention to Temperature
Fish are like Goldilocks—too hot, too cold, they’re not happy. I’ve seen people keep angelfish with goldfish and wonder why things aren’t working out. Spoiler: it’s the temperature.
Keep it simple:
- Get a heater (unless you’re running a cold-water tank). If you’ve got multiple tanks, look into heating the whole room instead—sometimes cheaper and easier than buying a heater for every single tank.
- Use a thermometer. Eye-balling or checking the temperature by feel it isn’t good enough. Experienced hobbyists are able to do that, your jedi tricks are not developed so use technology.
- Check what temp your species actually prefer. Tropical? 75–80°F. Goldfish? Cooler. Don’t mix ‘em. The caveat is, however, that some fish ARE more tolerant as long as the temperature does not fluctuate. Research. Research. Research.
7. Overfeeding
It’s fun to feed fish. It’s less fun when they end up with bloated bellies and your tank smells like old leftovers. Plus, all that uneaten food turns into pure nastiness. I always have feeder snails because I overfeed, check my store if you need any... hint, hint.
Pro tip:
- Only feed what they eat in a few minutes. There are exceptions, like mystery snails or shrimp, but don't throw a whole zucchini in and expect it to be gone in an hour.
- Watch how much is left over—if food’s sinking to the bottom and just sitting there, you’re overdoing it... unless you have bottom feeders, with mid-column or top feeders. Floating food will hang in the water for a bit, but sinking pellets are meant to head straight to the bottom for those little corys doing cleanup duty. Balance it out.
- Scoop out the leftovers. Don’t let ‘em rot in there.
8. Skipping Quarantine
Adding new fish straight into your main tank without quarantine is like inviting guests into your home without checking if they’re carrying anything contagious. Not a great idea. Also, even if you trust the source it's better to be safe, than sorry. I have an adult, sarcastic reference about being safe, but I'll just leave that here for you to ponder.
Save yourself the headache:
- Set up a small quarantine tank. It doesn’t have to be fancy. Sponge filter, heater, sponge bob's pineapple and a plastic tree will do it. You can keep the sponge filter in another tank when you do not have fish in quarantine, that way you always have a cycled filter - or put a mystery snail in there for when no one is staying in Hotel Q.
- Keep new fish isolated for about 4 weeks. Here’s my usual routine: first week I just watch them closely, second week I add some aquarium salt, third week I treat with the med trio, and the fourth week I watch again. If they show any signs of illness, rinse and repeat the process. I do it this way because they need time to acclimate to YOUR water, and throwing a bunch of meds at them right away is the equivalent of buying all the cold medicine, flu medicine, digestive medicine for a broken fingernail: completely unnecessary, and the overkill will make you feel like crap.
Watch for my blog about quarantining for more info.
9. Over-Cleaning Your Tank and Over-Analyzing Your Fish
If you’re a new fish parent, this one’s for you. Parents will totally get this—new baby comes home, and you and your partner take turns watching that tiny chest rise and fall, making sure the baby’s still breathing. Nothing rational about it, but you gotta check. Biggest fear? That they’ll just stop. Don’t judge me—I do way weirder stuff, like keeping baby teeth and then getting called out by friends. But that’s a story for another blog (in a non-fish forum).
Same thing happens with fish. You’ll stare at them, watching every little twitch or bubble like you’re waiting for the apocalypse. Chill. Fish don’t need a constant FBI surveillance.
Now, over-cleaning your tank? Also a rookie move. If you’re rinsing your filter under the tap once a week until all that brown gunk is gone—congratulations, you just flushed out all your beneficial bacteria down the drain. Those bacteria are the good guys, the real MVPs keeping your tank healthy. They do not live in the water column, so cleaning the algae off of the pineapple weekly is doing more damage than good.
Here’s the deal: gravel vac when it’s really nasty. Clean your filter every few months—not weekly. If your water’s cloudy, don’t panic; add more filtration or do a water change. But don’t go full June Cleaver on your tank. Let it be like a teenager’s dirty room—kinda messy but still livable.
10. Waiting Too Long to Say Goodbye
I (as will you, most likely, someday if you don’t follow number 8) have had some nasty fish diseases—think fish TB and other horrors. Read my blog about fish diseases for more details and all the stupid mistakes I made.
Here’s the hard truth:
- Learn humane methods of euthanasia. It’s not fun, but it’s part of the hobby.
- Ask an aquatic vet if you’re not sure what’s going on. Yes, they exist. Yes, it's expensive. Apparently, there's a box store with pet insurance that covers fish. Overkill? For that Tetra, yah... but for that $1,000 Silver Arawana that you caught yourself... worth it.
Wrapping it up:
Keeping fish isn’t magic—it takes time and patience. The Doctor cannot bring you back to change the timeline so you do not overfeed Goldie, but a little knowledge and research will help you to not be a Dalek exterminating everything. Cycle your tank, pick your fish wisely, don’t freak out over every bubble, bring a towel, and remember—wash your frickin' hands before AND after you put them in the tank. Good luck!