I'm at my wits end and about to give up.

[FastFly67]

25 gallon tank. occupants were 1 originally koi. they've been moved and within days replaced with a crowntail betta. after (roughly) a week, 9 danios were added. Lost 1 that night (wrote it off due to stress of move, it was a small one) by the end of the week all of the others were gone. they were all pretty much white (not dots like ich, almost like a chemical burn) i'd been doing water changes every other day. replaced the danios last night with 10 platies. came home to find 3 dead (same white "film" but it doesn't cover their entire bodies" and the rest pretty much chilling at the bottom) the beta is completely unfazed by all of this.
temp is 78
ammonia is 1 ppm (with three dead fish that were in their for close to 8 hours i'm not suprised)
nitrate and nitrite both at 0
oh is 6 (or less, that's the lowest it reads)
filter is a marineland penguin 150 w/ biowheel
and i run a bubble bar
what's going on
unfortunately i don't have a camera to take pics
i'm tired of losing fish

 

[Zagz]

Are you buying all the fish from the same store?

 

[FastFly67]

Are you buying all the fish from the same store?

Nope. Danios came from Petco and the platies came from my work
and the betta was bought ages ago at a petsmart iirc

 

[Zagz]

OK thought it might be the fish store. It sounds like a bacterial infection, perhaps columnaris. I would treat the tank with Jungle's fungus elliminator as it works for both fungus and bacterial infections. Don't add anymore fish quite yet. Do a water change and take any active carbon out of the filter.

 

[FastFly67]

OK thought it might be the fish store. It sounds like a bacterial infection, perhaps columnaris. I would treat the tank with Jungle's fungus elliminator as it works for both fungus and bacterial infections. Don't add anymore fish quite yet. Do a water change and take any active carbon out of the filter.

Off to the fish store I go. How long should I leave the carbon out of the filter? It's part of the pillow filter so that'll just leave the bio-wheel on there. Should I add something to the filter for the "mechanical" filtration?

 

[Zagz]

As long as you have medication in the water carbon needs to be out. Get some filter floss to use in the meantime. I use Walmart brand crib 100% polyester batting.

Now you will need to keep an eye on the parameters as the tank is going through a recycle. The ammonia spike from the deaths of the fish can cause a nitrite spike soon after.

 

[FastFly67]

update
they are doing SO much better. thank you so much for you're help zags. i figured it was my screwing up and somehow killing the cycle. turns out it was fungal or parasitic or something. how long should i wait to add new fish? i've got 4 of the original platies left (the betta was never even affected by whatever was knocking off the other fishies) but it's 2 females and 2 males. now i'm afraid their going to start picking at a particular female.

 

[Zagz]

I`d give it a week unless the picking at the females gets too much that it stresses the females out too much. When adding new fish make sure to add only a couple at a time so as to not cause an ammonia spike.