Neon Tetras and filter problem?

it might be an isolated incident. note that neon tetra are schooling fishes, they need to be in group of 6+ i generally don't keep any less than 12 of these.

another thing to consider is that neon tetra live is relatively low pH compare to most fishes. you should check your city's water supply. it should be as close as you can get to 6.8.

you might want to raise the temperature up to 76 though. while 70 is accepted, it is almost the bare minimum. plus let's say during the day, your house is 77 degree, the water would be at 77 too. but come night time and y our house become 70 degree, now your fish just experience a 7 degree temperature flux. this is bad for most fish, stability is the key. further more. they digest much slower at 70 degree, this can cause over feeding even if you don't notice it.

adding 4 fishes at a time in a 10 gallons tank is risky for biological spike. just to be safe, in the week that you add any more than 3+ fishes, you should do a 10% water change every 3 days.

one least thing i want to comment is 20 neon tetra in a 10 gallons tank is not over stock. the 1 inch of fish per gallon is total crap. it've been around since the 70s. our biological filtration have evolved much since. stay on top of your water changes. i house 30 neon tetras and about 20-30 red crystal shrimps in a 20 gallons long aquarium. no death

if the other fish look healthy then there's probably nothing wrong with your tank. It might have been sick from before you rescued it. I'm guessing that you recently set up your own aquarium. New aquariums are delicate, since they do not yet have a strong bacterial colony, and even if it's not new, you shouldn't be adding more than 3 fish at a time to an aquarium. Take it easy, buy one or 2 fish every now and then and don't overstock. You might also want to buy a bottle of bacterial supplement, and use it according to the instructions when you add a new fish.

Number one: a ten gallon tank is not nearly big enough for Neons. Yes, they are small fish. However, they swim very fast, and need plenty of room to do so.

Number two: the temperature is way too low! Only cold-water fish would be happy in there! Neons will be much happier if the water is 76-78 degrees.

Number 3: Neons are extremely fragile. They do best in mature, established tanks that do not have fluctuations. New tanks have all sorts of fluctuations and so many new hobbyists want Neons. They nearly always die. They cannot tolerate fluctuations in their water quality.

One of these reasons is probably why your fish died.

8-10 Neons would be a good school, in a 20 gallon long (or larger) aquarium. Not in a 10 gallon.

Good luck!

The fish probably died/was dying before it got stuck to the filter, it wasn't the filter that killed it.