This is odd

[Obsidian]

Recently when I change the water in my tank the next day the water will be mildly cloudy and there appears to be a thin white film over all of the decorations. It is not a powder or anything like that and it is most evident on gravel that is actually being vacuumed. In the corners behind the decorations, where I cannot reach, the gravel appears mostly normal.

Typically I do 30-40% water changes in this tank every 1-2 weeks. Yesterday I completed 2 30% water changes about 2 hours apart due to having one fish that appears to be wasting away. 2 weeks ago I lost both Oto cats in this tank with similar symptoms (I pulled both from the tank before they died but that did not help). The cherry barb with those symptoms cannot be found so I am guessing she died. I may have to tear the whole tank apart to find her. With the advent of her symptoms I now believe "something is going on here." The other fish appear well nourished and normal in their behavior.

When I pick up a piece of the gravel and rub it there is a film that does come off with rubbing. I took pictures but I have something seriously wrong with my ability to do anything with pictures right now, which I believe is the result of an update gone wrong. Lets all be shocked together. Currently, if I attempt to host the pictures on photobucket it shuts down my firefox. Thanks MS.

2 weeks ago when I performed the water change after the losses the water resumed its normal coloration and the gravel appeared normal inside 2 days.

Parameters:
Ammonia: readable but less than .25 (I believe this is because I can't find the missing barb)
Nitrite: 0
Nitrate: about 8 (more than 5 less than 10)
Ph: 7.2 This is a drop from 7.4 last week.
Temp: 80
Filter: AC50- Rinsed internal sponge out 2 weeks ago in old tank water.
Tank is 20 gallons.
I feed once daily top food and once daily bottom food. Varied diet of flakes, freeze dried blood worms, shrimp pellets, and algae wafers. About once a week they get a slice of cucumber. On occasion I feed skinned peas.

Any insight would be welcomed. I am completely stumped.

[Obsidian]

No ideas at all?

[Obsidian]

I am finally able to post a picture of this. If anyone has any idea it would be greatly appreciated. I can only solve it if I can figure out what is going wrong.

[trashion]

Weird! My only guess is maybe some kind of mineral buildup?

[Danh]

Test the ph of your tap water? Maybe it's fluctuating too much?

[emc7]

I don't know either. I agree that it looks more like a mineral precipitate than a fungus or algae or bacteria. Do you know anyone who can do a comprehensive water test? I've been getting off-white crystal-like stuff on my heaters, but it doesn't look like that.

[Danh]

It kind of looks like my 20g recently. I only had some plants in it... I had just fed my 450g and had catfish juice and oil all over my hands, so I rinsed them in my 20g. It was pretty nasty the next day... Is there something dead i the tank?

[Obsidian]

Nope, nothing dead in the tank, and it goes away 2-3 days later. It feels mildly slimy but I don't know if that is just the natural slime coat that is on the tank in general or not.