Hi,
My fish keep dying on me and I just can't figure out why. Three now in the last four days.

I have 20 gallon high tank, had 5 blackskirt tetras, 2 white cloud minnows, 2 danios, 1 rummy nose tetra. Tank is planted with java fern and hornwort, been running about three months. The last fish, 3 blackskirts, were added about two weeks ago. I found the white cloud dead four? days ago, he must have been there awhile wo me noticing because it was moldy and looked like it had been half eaten. White cloud died the day after, it seemed healthy, eating, day before. He had been hiding a while before that, I thought the new tetras might be scaring him. The danio died today, suddenly with no symptoms. It looked like he had hemmoraged inside The minnow and danio were both missing their tails when I found them.

The other danio has been hanging out at the bottom, not eating. Earlier today the two were hanging around the intake of the filter, which I thought was a bad sign. The five remaining tetras seem healthy, I dont know if the danio will live thru the night

So anyways I have been checking the water constantly, after every fish died. There is no reading for ammonia or nitrites or chlorine. Tested for nitrates the other day with dipstick test, I got what might have been a really really low reading, it barely registered if it was there, the dipsticks are kind of subjective. I did a 20 percent water change tonight.

So if there is nothing wrong with the water, what is causing this? At no point since I've had my tank have I gotton an ammonia reading with this testkit (its Junglelab); is it possible the kit is faulty and there is ammonia? I suspect there is a pretty high phosphorous level in the water; my tapwater is naturally high in phosphorous, I tend to get a lot of brown diatom algae. I also didnt realize I needed to clean under the lava rocks in my tank. So a week ago I took the rocks out and cleaned three months worth of poo and debris out, it was pretty gross. The gunk build up has probably contributed to high phosphorous. Could high levels hurt the fish? I was careful not to overclean the rocks, I rinsed them in tank water, I only replaced about 20 percent of the water, so as not to kill the bacteria.

I tried to be careful not to freak the fish out, although the rummy nose has always stressed easily. He was pretty unhappy.

About two weeks ago I took the filter apart and cleaned it out. All I did though was gently rinse the propeller and filter in tank water, and wipe some of the brown gunk off. I tried to be careful not to overclean anything.

The only other thing I can think of, is I've been treating the whole tank for internal parasites for the last month, with junglelab medicated food. I did this because there was a lot of white poop and the smallest tetra seemed kind of skinny. I figured it couldn't hurt to worm them all just in case. Unfortunatly the big tetras ate most of the food, the smaller fish got little to none. So if they did have parasites the medicine may not have been effective for them. Would parasites cause such sudden deaths?

So after all that, does anyone have an opinion what the culprit is? I really need some help, my fish are dropping like flies.

Thanks so much, you people are great.