Ultrastructural observation on attached Apiosoma did not reveal any interference with the host cell serving as substrate (Lom and Corliss, 1968; Lom, 1973; Fitzgerald et al., 1982) or peripheral tissue response. Thus, mortalities following massive colonisation of gills by sessilians (Fijan, 1961) could result from the dense cover of sessilians disrupting gas exchange through the respiratory epithelium. The only exception among these infections are the colonies of the stalked sessilia Heteropolaria (Epistilis) which cause lesions (“red sore”) at the stalk atachment to the fish skin, these inflamed haemorrhagic lesions are also contaminated with the bacterium Aeromonas hydrophila (Esch et al., 1976; Miller and Chapman, 1976).
Abstract: Epistylis sp. is an emerging parasite in Brazil and is commonly found in surubim Pseudoplatystoma spp. during the initial stages of production and, therefore, requires the establishment of prophylactic and therapeutic measures, aiming at its control. Therefore, the present study aimed to evaluate the use of common salt to control Epistylis sp. in surubim juveniles. The fish (n = 48) were randomly divided into three treatments, and a control group, with three replicates each. The treatments consisted of three therapeutic baths performed every 48 hours for 20 minutes with different concentrations of common salt: 0.0%, 1.0%, 1.5% and 2.0%. The experiment lasted six days. Twenty-four hours after the last bath, the fish were sampled to check for the presence of the parasite and blood collection. Salt
treatments of 1.5% and 2.0% were effective in eliminating parasites in 82% and 78% of treated fish, respectively. Fish subjected to the 2.0% salt bath showed higher hematocrit and hemoglobin values (P <0.05) compared to fish in the control group, in addition to changes in leukocyte values. Common salt can be used to control Epistylis sp. in naturally infected surubim juveniles.
I had never encountered nor heard of this problem before this post. So I went looking for the science as is my habit. The above is one piece of info I found.
In reading I discovered that this infection, on bottom dwellers tends to start on the percoral fins as they are more in contact than the substrate. Also this parasite tends to grow a bit away from the body with a tuft which often causes it to mistaken for a fungus as fishorama pointed out. Early on it can be mistaken for ich as well.
I also read that ich tends to spread all over a fish while epistylis tends to concentrate more in fewer areas.
Rodrigues, R.A., dos Santos Sanchez, M.S., Fantini, L.E., Nunes, A.L., Kasai, R.Y.D. and de Campos, C.M., 2019. Use of common salt (NaCl) in the control Epistylis sp. in hybrid sorubim. Revista Agrarian, 12(46), pp.495-502.
https://pdfs.semanticscholar.org/97cd/b05467e84c54052374ffa4ee0ec7e07709d7.pdf
I think the fish has ich from what I can see in the pic and what I have now read re Epistylis sp. However, if the salt article is accurate, then it would appear help with either ich or Epistylis.