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Scatophagus argus - How long can you keep them?

by W. Hering
of Cape Town, South Africa, c/o The Calypso Fish and Aquaria Club, London, England.
Aquarticles

 Do you belong to the small group of lucky aquarists who can keep these fish without any difficulty for years, or is your mortality rate so high that you are bound to buy new ones every few months to give them another try?  If you are a member of the latter group - which,  in my experience, is in the majority - it will perhaps pay you to consider to keep them in the way I do.

   Scats are common inhabitants of estuarine waters of Malaysia but also occur as landlocked populations in fresh water far from the sea. According to information I have had, most of these fish are caught in brackish water along the coast, thus saving the expense of long transport to sea or air ports. Until the time of shipment they are kept in natural brackish water, but afterwards they are transferred to fresh water, sometimes with added salt, sometimes without. No wonder that quite a number of them are already in a bad state before they land in your tank.  
   If you are lucky, you might unwittingly become the owner of scats caught in freshwater. In this case you probably will have no problems at all and are even able to give good advice to your unlucky fellow aquarists, unfortunately not helping them very much.
    It has been said that scats must he kept in hard, alkaline and slightly  saline  water:  for this reason it is commonly advised to add some table salt to the water. Nothing could be more wrong, because most of the scats are not dying because of low salinity. Aquarists often find that the addition of salt does not have any beneficial effect on their fish at all.

Mostly the mortality is due to two factors, viz.:

1. The wrong percentage ratio of magnesium  to calcium water.
- In sea water: Magnesium approx. 1,294 p.p.m.,  Calcium 413 p.p.m.
- In local (Cape Town) average tap water: Magnesium 2 - 5p.p.m.,  Calcium 9 - 15p.p.m.
   Therefore  I  rectify somewhat by increasing the magnesium content by adding two teaspoons of Magnesium sulphate (Epsom Salts) to every four to five gallons of tap water.

2.   Phenol poisoning.
  Phenol is a slow acting nerve poison. Symptoms, therefore, are: nervousness, convulsions, the fish dart wildly around the tank and are very frightened. In fact they may become so agitated that they ram their heads against rocks and glass, and even shoot downwards at high speed to bury their heads deeply in the sand. Phenol formation can be observed even in well kept tanks four to eight hours after feeding. Scats are far more sensitive to it than any other fish and accumulate the poison in their tissues.
   To prevent phenol poisoning I use an activated charcoal filter. One pint of this charcoal in a five gallon tank occupied by two scats remains active for four to six weeks.

 "Pressure Disease".
   Further, mention must be made that in the first weeks, before the fish are fully acclimatised, they may  suffer  from  the  so-called Pressure Disease. In this condition  the tissue and blood of the fish are flooded with water because of the difference in osmotic pressure between  the  inner  and  outer medium.   

   The process of osmosis regulation in brackish water fish is complicated and I must admit that very little about the nature of this mechanism is known.

In simple language it could be explained as follows:
   Marine fishes lose water constantly by osmosis. They replenish this  loss  by  swallowing  large quantities of sea water. In their intestines the sea water salts are absorbed and excessive salt is excreted by special cells in the gill epithelium, the so called chloride cells.
   Freshwater fishes practically do not drink water at all. Instead of the "chloride cells" they have mucous filaments in their gills which can absorb water and salts from the outer medium (the surrounding water). Excessive water and salt are excreted through the kidneys.
   If you put a marine fish into fresh water it will not stop its habit of drinking water. Brackish water fish behave like marine fish, but  when  transferred  to  fresh water they stop drinking this and slowly their "chloride cells" degenerate  and, let's say, "water cells" appear as in freshwater fish.  

   The art of acclimatising brackish water fish to fresh water, therefore, is to synchronise the dilution of the salt water with the gradual disappearance  of  the  "chloride cells" and the formation of the "water cells". 

    In   their  natural  environment scats regulate their osmotic pressure in a very ingenious way: they eat the excrements of other animals, preferably of those with a high urea content, like in ducks and other birds. Francis Day, in "Fishes of India" (1878), said: "I have opened many specimens and of these taken near inhabited locations had as a rule their stomachs full of ordure."  Furthermore they scrape stones, etc., that are overgrown by certain algae - in Malaya called "lap-lap" - blue-green algae of high nitrogen content.In an aquarium they eat their own faeces, and those of other fish.
   Now a phenomenon occurred which at first we could not explain. If scats were fed on the excrements of freshwater fish they always died in a short time. The reason for this may be that freshwater teleosts excrete practically no urea but mostly ammonia, which is poisonous.
   Therefore, before the fish are fully acclimatised, we have to keep them separate and not together with other fishes. If scats are put into distilled water and fed with urea they survive this treatment for about a week, whereas without urea they die within hours.
   To feed scats with urea is a tricky business since an infinitesimal amount too much will poison them. Looking around for a substitute we found an excellent food in ox kidney. The kidneys must be frozen first, otherwise they are too difficult to digest. Care must also be taken to avoid fatty parts. I have fed scats over the years on nothing  else  but  kidneys and algae.
   Filamentous algae I collect once or twice a year in freshwater pools around Cape Town. I hang them over string in a shady place to dry them, and packed in plastic bags and stored in a dark room they keep their green colour for months.  The best algae to feed are a euryaline species called Enteromorpha which grows here in a small brackish water river on the way to Strandfontein in huge quantities. These algae have a high vitamin B1 content. The dried algae are first soaked in fresh water (not in aquarium water) where they swell up immediately and cannot be differentiated from fresh algae.

   For one or two young scats I use a four gallon tank without any gravel. If the bottom is of glass I paste  black  paper  underneath. After filling the tank with the magnesium tap water and connecting the charcoal filter. I aerate the water strongly for one night. If available, some old tank water should be added. Then I put in many floating plants, such as Elodea, etc. Evaporated water is replaced with fresh water without using magnesium. Before the fish can be transferred into an ordinary tank together with other fish visible growth must have taken place.
   Scats like a dark place to hide. A slab of black slate leaning against one of the glass walls will be occupied immediately as a sort of "garage". The fish must be able to swim in and out on either side. If you have four scats you must provide four separate "parking" places for them: each prefers to have a "garage" of its own.

   All my scats died after about four years in captivity because of a hardening of eggs inside them (egg bound) because all the scats I had turned out to be females. I still have one huge female left, going now into her sixth year. It is a pity that I was never able to come across a male scat, and sometimes  I  think  that  only females migrate to brackish water.  This has its parallel in Monodactylus: I never found a male in brackish water. I opened at least twenty of them: all were females.

   In South Africa there occurs one species of the Scatophagidae family, viz., Scatophagus tetracanthus.  I caught two at Richard's Bay. Natal. They have bars. not spots. But I failed to acclimatise them to fresh water - they tolerated a density of l.008 at 20~C, but died at 1.004.