About two weeks ago I noticed some white gunk floating in my aging water reservoir. Take a look closer and there are small white colonies scattered all over the bottom of the tank. I thought I thoroughly cleaned it out but it has returned.

The reservoir is an old 50 g tall I had laying around. For circulation I have an air stone. No top either is is open to the air. Once filled, it sits for a week total and during that time I change the water for all the show tanks.

I feel like this might be something to worry about but how long did it go on with out me noticing the colonies?!? Have you guys ever seen this? I'd hate to have to keep cleaning out the reservoir every week before I fall it. Water change is today but is the reserved water okay to use (said with a grain of salt) ?

Colonies of what? Are they colonies of actual living creatures? Can you look with a magnifying glass?

Why do you age the water anyhow?

I call them colonies because they look similar to bacteria colonies but I don't know if they are. They're very small white mounds that have a fuzzy halo. Maybe it is mold? Idk.

I age my water because it is supersaturated with dissolved gasses. When the tiny bubbles eventually gas off my pH drops. Straight from the tap the pH is usually around 8.6 - 9.0 and after 24 hrs. the pH has dropped to around 7.2 - 7.6. If I do a water change straight from tap, the fish suffer (maybe "suffer" is extreme but anything bad that happens to them is suffering IMO, mother bear syndrome). They get red ulcers on their faces and their slime coats thin out. I attribute this to pH drop. They are my prize Oranda goldfish, can't loose them.

As for what I'm going to do with the reservoir tank. I'll clean it out again and let some bleach sit in there for a while. Are there any recommendations for concentrations?

I'm thinking maybe the flow at the bottom of the tank is too stagnant and it's allows some type of creature to flourish. Right now there is a small air stone and pump that breaks up the water but the tank is a 50 tall. I'm going to replace it with a spare hydro pump I have (I think is is rated about 300 gph, something like that).

I use a 90 gallon with a top for a water tank. I have two air stone in mine. I get algae to grow once in while. For me I pump the water down to couple inches and then siphon the bottom out about 3 times of year. I would recommend a top for that tank. My water comes out about 7.9 and then falls to 7.4 in 24 hours and that RO water.

I have seen what you are describing. If I understand what I have read its a type of algae and or a type of bacteria that like the dark undisturbed water. I had to add a aerator to get it to stop. I guess its also a pest for he city water supply and is one of the reasons chlorine and other junk is added to our drinking water to help stop the water from making people sick.

Because of the PH drop I also do this. But since it happens in 24hrs, that is how long I allow the water to sit. No problems this way.

A closer look may be needed to find what it really is that you are seeing. I find the bottom of my reserve water barrels does collect a lot of white stuff but it is just hard water deposits rather than something growing. Any chance of that? Grabbing some, does it feel slick, slimy or more gritty like deposits?
I find just running the hose back into the reserve before adding to the tank gets this moved out so that it doesn't collect. But then I would also assume that your water and mine are quite different.

Dust.... when I cleaned everything out it look like dust coated in a white film. Similar to what happens to air tubing when left in water. It does feel slick and slimy. But whatever it is, it took the opportunity to proliferate on the dust. Looks like I'll be getting a cover.

Thanks everyone for your help.