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Brooklynella hostilis is found as a parasite in Marine Aquaria far more often than is commonly recognized as it tends to be overshadowed by the more frequent & well known parasites viz. Cryptocaryon irritans and Amyloodinium (Oodinium) ocellatum.
it like so many others is a ciliated protozoan which in many respects resembles its counterpart in fresh water Chilodonella ( see previous article in Aquarium net ). When conditions become favourable to its reproduction, very rapid multiplication takes place , reproduction occurs by simple binary fission, & such massive reproduction can & does cause fatalities , which is brought about by severe weakening of the host fishes.
The literature reports many differing species of fish as being susceptible, & the first reports of this parasite were related by accounts from some of the more prominent (at that time) Public Aquariums, such as the New York Aquarium Steinhart, etc.
Typical signs of infection
Brooklynella hostilis
Water
There is little doubt that fish exposed to lowered water quality , and in particular the stress of elevated Ammonia /Nitrite levels such as are brought about in shipping, can induce an outbreak of this parasite. All the literature seems to confirm this.
Behaviour
Fish demonstrate lethargy, will "toy" with their food, appearing to eat & then spitting it out. Respiration becomes difficult as the Gills become heavily parasitized , & can easily be observed.
Brooklynella hostilis infestation on a Damsel fish.
Note typical faded body colour, as well as severe erosionof the Dorsal & Caudal fins.
Body
A "faded" appearance of small areas becomes apparent , & such areas spread outwardly as the infestation progresses. Sloughing of the epithelium will occur in later stages.
Gills
Gills will become massively parasitized and a smear or other examination of the gills should easily determine the cause of the infestation.
Skin
As the "sloughing" occurs increasingly large areas of damage can be seen, as the skin becomes broken down by the parasites activities.
Histo-Patholgy
Parasite can be confirmed by skin or gill smears from suspected fish. Parasites are mobile and range in size from 60x80 microns to 40x48. In shape they are heart or kidney shaped ( see illustration), and they have typically ventral organ for attachment to their host.
Treatment
The literature has very little on effective treatment, & the authors disagree on the application of Copper as treatment, (Stopskopf's book recommending its use, whilst Blasiola stating categorically it does not work). In this authors experience I have to agree with Blasiola, I have never found it to be effective against Brooklynella.
The combination of the following treatments & techniques have worked for me, on several occasions with varying degrees of infection, to eliminate the parasite, but I have had to employ at least 2 of the methods, & often all four. With careful attention the parasite can be brought under control( eliminated), but one should be aware there is no " 24 hour" simple cure. Be very suspicious of anyone telling you that they have such a remedy.
1) Giving a "dip" in freshwater of the same temperature & pH as the Marine tank, for about 15 minutes ( careful observation must be made, during this time, to avoid distress, & the fish removed, if major problem is observed).
2) Giving a bath in Sea water with Formalin added add at a dosage of 1000 ppm for some 15 minutes.
3) Adding an Acriflavine product (such as Fish-Vet's Revive) to the tank water for a period of 2 weeks after the above treatments.
4) Taking severe steps to ensure that water quality is optimum along with the TOTAL removal of any detrital matter .
I would mention that G.Basleer mentions in his book that Quinicrin gives a good result, regrettably he does not give any figures for dosage, or any contraindications.
If any secondary infections with by bacteria are observed then the use of an antibiotic would be useful. The use of UV to help prevent secondary infection should be employed.
It is useful to point out to the Aquarist who is intending to buy fish, that the judicious way to avoid to problems that this noxious parasite can bring to your Aquarium, is often best managed, by ensuring that your dealer has the fish in his tank for some 2 weeks prior to you taking it home. Most fish that will break out due to the stresses of transport , with this parasite will do so within this time.
To be fair to your dealer, you should show goodwill, by making it a conditional purchase, i.e. offer to pay for the fish, or at least part of it, provided he keeps it, & it is alright after the period of time mentioned has elapsed. Brooklynella is not an easy parasite to eliminate, so the effort to avoid it, is worthwhile.
I suspect that some of you as you read what would appear to be a depressing catalogue of parasites just waiting to attack your precious fish, may become disheartened somewhat, especially if you have had the misfortune to suffer one or more attacks in your early days in the Hobby. Therefore at the risk of been repetitive let me again emphasize that fish have an amazing ability to withstand infections, their immune system like most Veterbrates is well developed, & only when the conditions that we prepare for them or that they are exposed to, before we receive them are substandard, can we anticipate outbreaks which will adversely impact their health & our enjoyment of them.
Thus once again, everything you can do, to maximize the conditions in your Aquarium as well as ensure that you buy fish that have been properly collected, ( not with Cyanide for example), held in conditions at all stages of the collecting/shipping processes, can you reasonably expect to have little or no problem.
I can hear some of you saying to yourselves, "how can I know, how the fish are collected /shipped etc". This is not easy, but careful reading of the literature, observing how your dealer handles his fish ( does he make any effort at quarantine etc), asking about his suppliers, (does he import directly, or buy from a wholesaler, ) does the wholesaler quarantine the fish etc. All of this will give you a far better idea of what to expect with the fish you buy. Furthermore, you should carefully observe how many dead or sick fish are evident in the dealers tanks, good dealers, who buy from good suppliers, will have far less than those that do not. All of this will enable you NOT to eliminate the risk, but greatly reduce it.
And we'll see you next month.
Shawn Prescott.
HTH
Graham
Look at NeMo, he WAS the only fish in my tank! Everything else is in my signature! Here's a pic, he looked perfect when I dropped him in, but 2 days later he looked like this!!! 
