I have two aquariums. One is a 30-gallon freshwater aquarium with two freshwater fish, and the other is a 10-gallon brackish water aquarium with one fish. Once each week I do a 10-percent water change on each aquarium, and once each month I do a 50-percent water change with a gravel vacuum. Also, once each year I completely take down both aquariums and clean everything. The pH of both aquariums has been a constant 8.2 and ammonia has been unmeasurable. Lately, I have been noticing an odd white build-up (I think it might be lime) on the outside and, partially, the inside of both aquariums. This build-up is crusty and very hard to scrape off. It hasn’t been affecting the fish in any way that I can see, but it does give the aquariums an ugly appearance. I am wondering what these build-ups are, how I can get rid of them and how I can prevent them from returning.
You are probably correct in your assumption that the white build-up is lime (calcium carbonate). It’s what makes water hard. It also might be from a build-up of salt residue from your brackish water aquarium. If the film is on the outside of the aquarium, it is not difficult to get rid of. There are commercial products, such as Lime-Off from Jungle Labs, that are especially made for the job. You can simply pour the solution on a clean rag, rub it on the aquarium and then wipe the film off with another clean rag.
Occasionally, the film is on the inside and is the result of frequent low water levels. This film is more difficult to get rid of because the aquarium must be completely emptied, the product used and then the aquarium thoroughly rinsed to remove the cleaning solution. Then the aquarium can be re-established.