>very few types of algal blooms are harmful. Most of the famous algal >species that are harmful occur in the marine environment, on the open >oceans. I don't know of any examples of harmful algal species in aquaria. I don't disagree with anything Jonathan has said, but I do feel a word of caution is in order here, especially for the benefit of newer aquarists. Cyanobacteria are not algae, but they look and act much like some types of true green algae. They are commonly referred to as blue-green algae and sometimes as "slime" algae. They form a very dark-green (sometimes almost black) film that can spread out and blanket anything that doesn't get out of its way. Like true green algae, cyanobacteria also bloom in response to heavy nutrient loads, and in thriving tend to remove these excess nutrients from solution. The important difference is that blue-green algae produce poisons (known as cyanotoxins) that can and will kill fish and other aquatic life. Not to cause a panic, as physical removal of the cyanobacteria mats and regular water changes will protect most fish even in the face of an ongoing bloom ("infection"). However, left unchecked, the poisons will accumulate and the fish will eventually get sick and die. I'm not sure whether the fish die as a direct result of the toxins, or indirectly as a result of opportunistic infections attacking weakened fish. Probably either, depending on the age, species, and overall health of the fish in question. I do know that fish, clearly heavily stressed and "sick" in the presence of a heavy blue-green algae bloom, can make quite quick and dramatic recoveries when the bulk of the algae is removed and the water is 50-80% changed. One reason I felt I needed to speak up here was Bonny's statement that her tank is brightly lit. When I think of diatoms, I think of the brown film ("brown algae") that tends to grow in the back corners and other low light situations rather than under bright light. I may be wrong here. Others with more knowledge feel free to correct me. I do know that cyanobacteria can grow under fairly modest light, but can also thrive in bright light, even full, direct sunlight. >< º> <º >< Ron Naples, FL ------------------------------------------------------------------------- This is the apistogramma mailing list, firstname.lastname@example.org. For instructions on how to subscribe or unsubscribe or get help, email email@example.com. Search http://altavista.digital.com for "Apistogramma Mailing List Archives"!