What are things that will stress Guppies and LongFin Serpae Tetras?

Serpae tetras are notorious fin nippers. Having long fins makes them a target to the other serpaes, that is stressfull .

They should be in a school of 8-12. They should be in a 30 gallon long or larger tank. or 36 inches or longer to swim in.

Guppies should be in a group of 3 or more. If you have mixed sexes, they will be constantly breeding, that is stressful. You need to keep 2-3 females per male. Serpae tetras may go after the guppies fins, especially male guppies.

The tempurature needs to be stable 24-7.

use a gravel vacuum to do partial water changes. Keep the water clean, dont over feed.

Dont net the fish,

dont take the fish out of the tank,

dont add chemicals to the tank other than Water conditioner like prime.

Dont over stock the tank.

Tank lights should be of for at least 12 hours a day.

there should be hiding places.

Constant load noise, kids banging the glass, foreign objects in the tank - all stresseful.

Too warm

Too cold

Ammonia

Nitrite

Nitrate

pH too low

pH too high

pH that changes

Too many water changes

Too much current

Not enough places to hide

Nipping from the serpaes

Not enough water

Too much light

Too much food

Being alive in general

Temperature changes

Being caught

Addition or loss of fish

And many, many more.

Frequent water changes are one of the best ways to keep fish healthy and long lived