There is a clear misunderstanding in this thread concerning the harmful/detrimental effects of nitrate. Like ammonia and nitrite, nitrate is poisonous to all fish; the habitat waters of all species we keep in aquaria are so close to zero they usually cannot be measured, so fish have evolved to function without nitrate (if that makes sense). With nitrate, the effect is slower, and it depends upon the species, the level of nitrate, and the exposure time. Cichlids are especially sensitive to nitrate, and it should always be well below 20 ppm. If you consult the cichlid authorities now they will confirm this. Dr. Neale Monks advises that the main effect of nitrate is a slow weakening of the fish, and following that other issues which the fish would/should be able to easily deal with become serious and usually fatal in time. During most of his, especially the early stages, the fish will show no symptoms.
Nitrate in the source water is a problem for several members. Ten is not that high, but of course the biological processes in the aquarium will add to the nitrate, unless significant steps are taken. Our member
@AbbeysDad has battled high nitrates in the source water, above 40 ppm if memory serves me, and he has information on his blog as to how you can deal with this.
To the specific issue on the base of the dorsal fin, I will leave for our more experienced members (I have been lucky to have so few issues over 30 years, and I would only be guessing).
@Colin_T
