This is my first thread start here so go easy on me! And please excuse me if this is long winded, I will try to give you as much info as possible.
I have just got back into fish tanks after an 8 year sabbatical and have been working on my tank for the past 2 months or so getting it up to snuff. Everything was going great up until last night.
Last night I had a very scary thing happen to my tank which resulted in the loss of 3 fish in less than 30 minutes. I will let you know the timeline of the week to better assist you in an answer, if you can make sense of everything.
10 gal tank
Aqueon 10 filter
2 albino corys
4 gold tetras
1 swordtail (female)
3 small sword plants
Everything seemed fine though water had a yellowish hue.
added 3 cardinal tetras (petco)
added 2 more sword plants and 1 bamboo (petco)
30% Water change
Came home and filter was dead. Bought Fluval Nano filter. I added loose carbon and then custom cut a water polishing pad to the filter, so from the bottom up on the left hand side of the container there is black sponge, about 1 inch of loose carbon, and then about 1 inch of water polishing pad.
30% Water change
Added pH Correct 7.0
Tank was clearing up nicely, meaning the problem with the yellowish color earlier in the week was likely the crappy filter dying on me.
Saturday Evening: (Here is when everything started to go to hell)
Went to That Fish Place and picked up a few more plants. Micro sword and 2 other plants (sorry can't remember the name off the top of my head). Got home, changed the water polishing pad as it was gross. Planted the three plants. Added about 1 ml of Seachem Flourish plant fert, then walked away. It was probably about 8:30pm by all this was completed.
About an hour later (9:30pm) I noticed that the entire tank was starting to turn milky white and millions of little bubbles were forming everywhere. I wasn't sure if this was a side-effect of the fert or the new polishing pad, so I dismissed it. I also noticed that a few fish where skimming the surface of the water, and dismissed it as them simply chasing around all the little surface bubbles. I turned off the hood light and sat down on the couch for about a half hour. I walked past the tank and noticed a few fish still skimming the surface of the tank, turned on the hood light, and found 3 dead fish!!! Immediately did another 30% change and added an AmmoniaSafe tablet assuming that there had been some random spike in ammonia. Today the tank looks almost as clear as it was on Friday evening, less than 24 hours later.
So, what the heck happened? Was it changing the polishing pad? Possible introduction of foreign water to the tank from the plants? An odd tank cycle? Or this thing I just learned about called "Outgassing"?