I have an established 15 gallon freshwater aquarium, containing 2 platies, 2 dwarf gouramis, 1 cory cat and 1 Chinese algae eater (still juvenile, no more than 2 inches in length).
I use a Fluval U2 underwater filter, and have plenty of aeration.
I have been entirely unable to keep this aquarium stable for the past month or so, beginning with a sudden onset of cloudy water. The cloudiness appeared initially to be whitish with a slight green tint so I figured it was a minor algae bloom. I reacted with 10 percent daily water changes, with light gravel vacuuming, and decreasing the amount of time the light was kept on. The cloudiness still has not cleared up, and has now lost the greenish tint. It persists as a slight haze to this very day. A few weeks ago, I did notice that my aquarium gravel was quite literally falling apart (breaking open and releasing a white chalky substance upon vacuuming) which I have no doubt contributed to the water cloudiness, so I replaced the gravel (with full knowledge of the biological disruption I would cause) and replaced it with a larger river pebble substrate which even my cory appears to like.
This was only the beginning of my troubles. Shortly after the onset of the cloudy water, my former guppy developed what appeared to be a dramatic case of dropsy. He never lost his appetite (food remained one of his favorite things up until the last day), but eventually lost his energy and would hang at mid level wiggling in place the majority of the day. With knowledge of the damage being done to his liver and kidneys, I peacefully relieved him of his suffering. During this time, my platies (I had 3 at the time) became rather lethargic, spending their days resting in my floating plants (fake, I have no real plants). I soon put my single male platy to rest after he became thin and frail, and appeared to develop a few red streaks in his fins.
During this entire time, my water parameters remained great, ammonia at zero, nitrites at zero, and while I had the occasional spike in nitrate levels, I kept this under control with the daily water changes. I use Prime to prep my water, and have used Stability when a dramatic change is being made (larger water change or during my gravel replacement). I follow all package instructions.
Now a few more things have developed and I am growing increasingly frustrated with my tank situation. My yellow platy (female) still has a great appetite but appears to be wasting a bit. She is smaller in size than she initially was, and is still rather lethargic, spending many hours floating in the shade below my filter, though she appears more energetic than she used to be. My red platy (female) appears to have a distended abdomen and sometimes some white stringy poo. Her anal opening is a bit more obvious than I would think to be normal, and is whitish in color. They are currently being fed Jungle Labs anti-parasite food (following all instructions on the bottle), as these appear to be obvious symptoms of internal parasites. The red platy is energetic most of the day and has a great appetite.
Most recently, my algae eater has developed the strangest of all manifestations. It appears that he has scraped the skin right off of his face. Now, he can get a little frisky (not aggressive in his young years, but he is full of energy and zooms around the tank at top speed frequently), so I suppose this could be an injury, but I have never had sharp enough substrate or plants to cause this. It is as if his scales came right off and just about his entire nose is pale in color and scaleless. He is chasing some of the other fish around more, but that is the only indication that he may be in any discomfort.
My gouramis (who were the newest edition to the tank) are not showing any symptoms of illness, nor is my cory cat (who is the oldest in the tank, coming from my previous guppy tank).
Any ideas or suggestions would be appreciated. I am aware of cycling my tank and keeping my water within appropriate parameters, and common beginner mistakes.
Thank you much!