Okay... Time To Fess Up... :)

[KirkyArcher]

Things did not end well for most of the goldfish.  The crayfish also died eventually, not too sure why.

Probably not enough goldfish as food for them?

 

[CoryFever]

Here goes nothing:
-Added ice to an aquarium in order to cool it down (too much in fact)
-Common pleco in a ten gallon
-Single discus with tiger barbs
-Raising plants outside, (for the sunlight) and forgetting to take them back in for the winter. The container froze completely
-Koi and goldfish in a ten gallon with a pleco
-Koi and goldfish in a ten gallon with tropical fish (10 neons)
-Severely overstocking you first tank
-Not cycling your first tank
-Losing $80 worth of fish because you did not dechlorinate your first tank.
-Buying a ten gallon as your first tank
-Introducing hydras to killifish fry.
 
There is more, this was merely an excerpt lol

 

[eduller]

I am fairly brand-new to fishkeeping, but I've done a lot of research since I started. This thread is actually very helpful so I don't feel like a demon fish-killer.
 

• Got a 5.5 gallon starter tank because it was on sale. And I thought I was doing well because I had gone there looking for a tiny fish bowl to put in my daughter's bedroom, so I felt like queen of the humane fishkeepers for getting such a "big" tank.
• Putting 5 glofish and 2 snails into an uncycled 5.5 gallon tank. I had enough sense to dechlorinate, but that's about it.
• Went out and got another glofish and a leopard danio the next day after 2 fish had died overnight. LFS guy told me "sometimes they just die from the shock of being moved." Lost 3 more fish by the end of that week.
• I was feeding the fish huge pinches of flakes 3 times a day because the LFS dude told me that glofish have a super fast metabolism and need to eat almost constantly. Plus an algae wafer a day for the snails.
• Got some weird amalgam fake cuttlebone thing with like honey and crap added to it instead of a real cuttlebone for the snails' shells. Clouded up the water.
• Took me forever to realize there was a little breakaway part on the tank hood that I could pop out and run cords and air tubing through instead of trying to shove everything through one little hole or have the hood on top of it, making the filter rattle annoyingly.
• Tank was smelly after a week (overfeeding much?), so I took everything out, washed the gravel with tap water in a used pasta strainer, wiped down the tank with vinegar, just totally changed out the filter media to a brand new one, tossed the fish and snails back into cold water without matching the temperature.
• Relied on all-in-one water test strips for a month.
• Burned up a heater by not realizing the suction cup had let go and it was sticking out of the water during a water change.
• Buried a plant rhizome and funked up the water from the rotting.
• Used a red solo cup and a gallon pitcher to do water changes for about 5 weeks before I discovered the existence of the water changer.
• Practically broke my arm for days trying to get the "self-starting" water changer to self-start before I just gave up and got it going the old fashioned way - one good suck on the end of the tube.
• Didn't realize my heater wasn't actually plugged in for about 2 weeks and wondering why the temperature wasn't getting above 73.

 
Somehow both of the snails and 3 of the fish have survived all this - including 2 of the original glofish I bought who have been through this entire ordeal. They are seemingly happy, peppy little guys/gals now. Their new (much larger) home is being prepared, but they've been really positive and upbeat through this whole thing lol. Now, thanks to all of you, I know at least the basics of what I need to do to keep these people alive.

 

[kiwifeather]

I did the same as everyone else with my first tank really. Non cycled, non tested 30L with a male betta, 6 glowlight tetras and 4 albino cories on sharp black gravel. Ouch. They were lucky in that they did get water changes and had live plants though I guess. And the water was treated. The betta survived with me for 2 years, and he was an adult when I got him. He moved into my 60L tank when I got it. I also moved into a flat before I got my 60L that had a large tank, which I'm guessing was at least 100L, but apart from my cories and tetras being moved in there was empty. I suggested a female betta group for it, and ordered 6 from a breeder. When they arrived, I plopped them all in only to notice one incredibly obvious male, and over a few days it became apparent there was a couple more males as well... Bought another tank with a divider to hold the boys till I could sell them. Tank had no lid and divider was flimsy. You can see where this is going. Boys kept getting into each other no matter what I did. I was lucky Atlas my first betta was incredibly placid in hindsight. Had goldfish in bowls as a kid too, which died a lot, or were eaten by the cat no matter how high the bowls went.

 

[LyraGuppi]

Hmmm...
Forgetting to accimate my cory to the new tank. He and I nearly had a heart attack!
Also, I was at Petco and asked the lady for two albino cories. She picked the big healthy one that I liked alot...and the small half dead one that I was praying she wouldn't pick. It died on the way home, which is forty-five min. away from Petco. Lesson learned: Speak up to workers!

 

[frapadoodle]

Hmmm...
Forgetting to accimate my cory to the new tank. He and I nearly had a heart attack!
Also, I was at Petco and asked the lady for two albino cories. She picked the big healthy one that I liked alot...and the small half dead one that I was praying she wouldn't pick. It died on the way home, which is forty-five min. away from Petco. Lesson learned: Speak up to workers!

Make the salespeople wait. I can spend a couple of hours inspecting tanks and watching fish. I I choose the exact ones I want, and will inspect them again as they are caught. Again when placed in the bag. If any are questionable I start over. Some salesreps. hide when I come in the store.
Lesson learned though. And always acclimate new fish.
You would be amazed at the blank stares when you tell the help at the chain stores that your fish usually die of old age.
As well get as many corys as your tank can handle. Nothing like the interaction of larger shoals.

 

[fluttermoth]

Agreed, frapdoodle!
 
Lyra; you're perfectly entitled to have the shop assistant catch exactly the fish you want. Okay, it might not be practical to do that if you're buying a dozen neons from a tank of 50, but for other fish, you get the individuals you want, not what the catcher wants to give you!
 
You wouldn't let them choose something like a rabbit or guinea pig for you, so don't let them do it with fish! It becomes even more important as you progress through the hobby and start looking at, and buying, more interesting and expensive fish.
 
I did get an odd glance in my LFS when I bought my red whiptail and I asked for "one that's not too skinny"; I knew what I meant 

 

[Baccus]

I remembered another mistake I have made a few times regarding tanks, I have managed to "cook" a few heaters. Usually by removing the heater and placing it on the glass lip of the tank and forgetting to turn it off while it was out of water. I have also "cooked" heaters during water changes and getting side tracked and allowed the heater to get high and dry when I had intended to not go lower than where the heater was sitting.
Another heater I killed, for some reason it was still on or warm and it went into a tank that had cooler water than the temp of the heater, needless to say the glass on the heater cracked and the whole thing needed throwing.
Hopefully now on a couple of my tanks I will not have any more "cooked" heaters because I finally invested in a couple of non breakable ones that also have built in safety cut offs if the heater is removed from water.
 
I have also nearly fried myself and my fish a few times by knocking lights into tanks

, all I can say is thank goodness for mandatory safety electrical cut off switches in Queensland houses.
 
And lastly not a huge mistake but still an annoying accident, adding fry endlers to my goldfish pond because they where hiding in a tangle off weeds I was removing from the tank and putting into the pond. Needless to say my pond  now has a healthy thriving endler population along with my goldfish.

 

[ficklefins]

Agreed, frapdoodle!
 
Lyra; you're perfectly entitled to have the shop assistant catch exactly the fish you want. Okay, it might not be practical to do that if you're buying a dozen neons from a tank of 50, but for other fish, you get the individuals you want, not what the catcher wants to give you!
 
You wouldn't let them choose something like a rabbit or guinea pig for you, so don't let them do it with fish! It becomes even more important as you progress through the hobby and start looking at, and buying, more interesting and expensive fish.
 
I did get an odd glance in my LFS when I bought my red whiptail and I asked for "one that's not too skinny"; I knew what I meant 

 
I work at a chain pet store and believe me, as long as you are polite about it, this does not get on our nerves!  I would rather deal with someone who makes me spend a half an hour catching the exact fish they want than getting yelled at by someone who isn't happy when I tell them they can't put 5 glofish in the 3 gallon tank they bought specifically for glofish (something that probably happens at least once a day).  Even though I love fish I hate working in the fish department because there are so many people who don't know what they're doing and most of them are the ones that refuse to let you teach them something.  "Well my cousin kept a goldfish in a bowl for 3 years so you obviously don't know what you're talking about."
 
Anyways, my god things I've done wrong.... mind you most of this was when I was 13....
 
1. Put 4 giant danios, 3 tiger barbs (who actually bred several times, strangely enough), two angelfish (who died within months), 4 random tetras, and a chinese algae eater in a 20 gallon tank (technically my parents did this because I was only 5 or 6 when we started out!)
2. Somebody gave me a 55 gallon tank and the first thing I did after filling it with water was go out and buy a baby black ghost knife fish the next day.  I knew absolutely nothing about the species and the guy at the store told me it ate fish flakes. 
 
Long story but this is probably my favorite:
3. I added a S. A. Bumblebee Catfish into my 55 gallon tank.  I loved that little fish, he would swim out and eat out of my hands when I was feeding my knife, and then he'd go hide inside of one of the fake tree stumps, hollow plastic ones that had small holes in them so they would sink.  He would go into the holes.  I hadn't seen him for over a month and I figured he had died.  I took one of the plastic stumps out to scrub the algae off of it and was running it under sink water to help get the algae off...... I was scrubbing it for about 2 minutes when all of a sudden out comes a little cat fish that struggled around for about a millisecond before going down the drain..... I felt sooooooo terrible. 
 
Extra fun fact, I got two more of those for my bigger tank because I liked them and they hid in the same decorations.  After THREE months of them never being around, I figured they were dead as well.  When the tank got a leak and completely drained, I found both of them flopping around on the bottom.  They were the only survivors, and they were about 3 times bigger than when I got them... not sure where the heck they were or how they were eating, but they were cool fish.

 

[ag-au]

My new biggest mistake was setting up the 10gallon (40L) up on the kitchen island. The island was wide enough, however, about 3 inches on each of the sides weren't supported. So, the sides of the kitchen island were slowly bending causing stress on the sides of the tank. You could literally look under the tank and see the other side. It was level to begin with, you couldn't see any light under it. It's still sagging, I'll try and post some pics if I remember.  I literally yelled "OH CRAP!" and scared my girlfriend when I noticed. she was like "are you okay!?" and I'm like "Well, for now. The tank's going to bust if we don't get a stand." 
 
 
SO ALWAYS LEVEL AND GET AN APROPRIATE STAND!! Peace of mind is priceless.

 

 

[frapadoodle]

Ficklefins, I would enjoy shopping with you.
Round here there are a couple of actual fishkeepers in the chain stores. They understand, and you can see the relief on thier faces when they find out you know what you are doing. And the sidewards glances when dealing with other customers are priceless.
I usually ask a few easy questions I know the answers to. If I am told a common pleco is fine in a 10gal., or oscars are community fish, I know I am on my own.
If they have to ask some else I still give them a shot, I appreciate the honesty rather than the make the sale mentality.

 

[NeonBlueLeon]

I usually ask a few easy questions I know the answers to. If I am told a common pleco is fine in a 10gal., or oscars are community fish, I know I am on my own.

I do this too!

 

[Blobfish]

I seriously electrocuted myself once by dropping the light ballast into a tank I had my hand in.  I could see a blue light flickering all around my hand and arm.  Fortunately I just managed to pull the plug in time.

 

[frapadoodle]

Okay to be honest, my biggest failure is getting Mrs. doodle involved in my hobby.
Planning an Asian themed tank. Pearl Gouramis, "They're ugly". Rasbora, "They're ugly". Khulis, which I want badly, "They're creepy". She has a snake phobia.
Oh but Neons are pretty. Ugh.

 

[Blobfish]

Okay to be honest, my biggest failure is getting Mrs. doodle involved in my hobby.
Planning an Asian themed tank. Pearl Gouramis, "They're ugly". Rasbora, "They're ugly". Khulis, which I want badly, "They're creepy". She has a snake phobia.
Oh but Neons are pretty. Ugh.

 
Sounds like a snakehead or two might be ideal for you