Brown, Mushy Spots on the Glass

Algae? That's my guess. I'm assuming the critters are all okay...?

My guess is brown algae which is usually caused by not having enough lighting and overfeeding. Look up brown algae in the search box you should be able to find how to treat it. Here's a link to start out with and give you an idea of what i'm talking about.

http://www.aquaticscape.com/articles/algae.htm#brown

It sounds very much like diatom growth.

which is usually caused by not having enough lighting and overfeeding

Click to expand...

In my tanks, it grows most prolifically on the most well-lit surfaces, which isn't indiciative of a preference for dim lighting levels. It also grows quite readily in my water-aging barrels, which certainly isn't indicative of overfeeding being an issue. I've also heard that it is only an issue in newly established aquariums, but I've got it growing in tanks that have been established for at least 4 years. The only way that I've found to control it in my own tanks is with Otocynclus catfish or manual removal (it wipes of very easily).