Fish keep dying, rogue homicidal fish or is my filter too powerful?

Question

I have a 36 gallon tank with an Aqueon® QuietFlow Aquarium Power Filter 55/75. as of now there are 2 red minor tetras and 4 neon tetras. I had another red minor tetra and 4 more neon tetras that all died over the last week. (There were a couple more that died the week before).

I used to have the 30 gal version of the filter, but it didnt seem to be able to keep up and the algae got out of control. I did a 50% water change, upgraded the filter to the 55/75 version and soaked all the decorations in bleach (and rinsed them off well after). It cleared up the water very well and the new filter seems to be working well.

About a week after I did the cleaning I got the new fish and since then they seem die off about 1 per day. Within a day of putting them in the tank, I notice that a couple of the neon tetras seem to have bits of their tails missing. Then about once a day I see a dead fish (almost always a neon tetra) thats sucked up against the Is this a case of an overagressive red minor tetra and they only get stuck by the filter after they die, or could my filter be too powerful and they get sucked up into it and not being able to swim away?

Edit: One thing I forgot to mention is that I got a water test done at PetSmart and they mentioned that the only issue is the Ph seems a bit low and I should do a 25% water change, could that be the issue?

Answer 1

Even though I think Trond's answer mostly covers this, I feel compelled to answer as well.

First off, just slow down here, in fact just stop. Don't change anything. You have so many factors contributing to what's going on here, you need to step back and take some stock in the whole situation.

Stop going to Petco, Petsmart or any other big box pet store, period. Find a good, locally-owned fish store and use them to get information and for where you get fish. Any person telling you your pH is off and suggesting you should change water as a result, should never give aquarium advice ever again, they should be doomed to clean up dog feces because they have no understanding of aquarium chemistry. Everything affects pH. You can open an door in your house or turn on a ceiling fan and measure the change in pH as a result. Unless you are deliberately adding something to change pH, like running CO₂, or trying to breed a super-sensitive fish like discus, completely ignore pH. It's a 10,000 ft measurement and completely meaningless for 99.9999% of home aquariums.

Next, make sure you fully understand the nitrogen cycle. Cycle your tank before adding anything. It is the bacteria that you build up during the cycle in the filter and the substrate and in the tank itself that actually keeps the water livable for fish. The initial cycle takes roughly 30 days under normal circumstances, but research it. You're trying to keep live animals, you owe it to them to understand how to setup a tank and keep them alive.

Third, algae are natural in a home aquarium. However, it thrives when there is an imbalance in the aquarium. The 2 main things that contribute to algae are nutrients (nitrate and phosphate primarily) and lighting. You control lighting with the type, quality, and the overall photoperiod the lights are on over the tank. You control nutrients through limited feeding and stocking and water changes. All of this is a balance specific to each individual tank. You overstock without increased water changes, you get too much algae. You overfeed, you get too much algae. You have either poor quality or too much lighting, you get too much algae. It's all a balance that every tank and keeper has to figure out.

Bleach is fine if you need to sterilize things. However, you need to fully dry those things, in the sun for a few days is ideal, or use a chlorine remover, before re-adding them back to the tank. Do not put bleached things back into the tank without doing one of these first.

Answer 2

Using bleach is the first and most serious problem in your tank.

Even trace amounts of bleach will kill the bacteria - the good and the bad ones.

When you get a new tank and/or a new filter, you need to cycle it to build up the good bacteria that break down the waste products. In a relevant article about nitrogen cycling on fishlore.com you could find how this is done.

Do not put fish in your tank before it has been cycled properly. You will need to buy a water test to see when the tank is properly cycled. Please see the mentioned article for the details about this.

A larger filter will not be a problem for your fish - quite the opposite, a larger filter will provide a larger surface for the good bacteria to grow on and will be able to handle more waste from more fish.

For now, you need to add the double dose of dechlorinator/water treatment (Aqua Safe or similar products).

Do not add any more fish before the tank is in biological balance (cycled properly).

Do learn how to do the water testing yourself - it is not complicated and you will need to do it weekly in the future.

Answer 3

Not likely your filter. I would never use bleach or any chemical on anything aquarium related. To clean, I've boiled loads of ornaments, rocks, wood and plastic plants, etc; I've melted a few plants but that's better than dead aquarium inhabitants. I've also invested in a set of dedicated brushes to clean small areas in the filers, tubing, et. Pipe cleaners also work. And I WASH HANDS & ARMS BEFORE I GO IN THERE. It was v. hard for me but I learned, from the day you start your aquarium nothing is going to be pristine clean. Brushes link: https://www.amazon.com/gp/product/B07DYQ21C4/ref=oh_aui_search_asin_title?ie=UTF8&psc=1