Who Gives A Fish Forums

I'm at a loss with this lump - anyone have any ideas what it could be and what the treatment might be? My water parameters all look good - 0 ammonia, 0 nitrite, and 20ppm nitrate. I've had the tank set up for about a year now and I do 50% water changes every week.

Anyone tried the DIY magnesium mix with Epsom salt and magnesium chloride? Way cheaper than branded products. I've been using the BRS recipe in my 180-gallon SPS dominant system for years. Just gotta watch the sulfate buildup if you're heavy on the Epsom.