Jim Graham just posted an interesting link on killietalk.

http://news.yahoo.com/s/ap/20060330/ap_on_sc/dead_zones

It could have a strong bearing on why the killifish sex ratios seem strangely skewed in SG, compared to much of our US experience.

I have probably almost never had exactly a 50:50 ratio of males to females, but my experience has been that the ratio is rarely very far from that. My water has, on average, been cooler than can easily be maintained in SG, and I use aeration and wads of plants in my baby growout containers. I also am close to fanatical not to expose babies to the slightest traces of ammonia or chlorine (or their combo, chloramine). All tend to damage gills and suffocate young fish. That indirectly could expose them to anoxia and skewed sex ratios. I also suspect them of causing bent spines and other deformities.

I usually hatch non-annuals in very shallow Petri dishes, quickly moving the hatched ones to a larger container, like a shoe box, with lots of Java Moss and some form of aeration/filtration. Annuals are hatched in larger plastic take-out containers, o/e, with a gentle airstone bubbling enough to break any surface film and stir the water slowly. I often toss a few sprigs of Java Moss in with the peat, but usually remove babies as soon as they hatch to a filtered and moss-filled larger container. Large surface area free of any film, and gentle aeration are vital.

I don't know if my cooler climate and hatching methods have shielded me from the problem, but if your situation differs, and you get bad ratios, I'd suggest looking to the oxygen thing as a first correction step.

As long as the water is in slow motion, any ammonium/ammonia produced by the fish or snails is quickly absorbed by the plants. I use plenty of light, so photosythesis is quite active.

You can spend over $50 on a fancy 3-reagent Lamotte chlorine kit that is hard and slow to use, and easy to misinterpret, or you can use the swimming-pool/spa kits that take 3 drops in about 10cc of water and turn yellow in less than 5 minutes if either chlorine or chloramine are present. I can usually detect less than 500 ppb with mine (held against a white refrigerator or card), and the reagent costs less than a dollar an ounce at Home Depot or other large chain stores with a spa dept.

You cannot test at home for ammonia, as less than 5 ppb has been shown to permanently damage baby fish gills. That is hundreds of times less than most aquarium test kits can detect. Keeping pH below 7.5 and using carbon filtration to remove chloramine are your best protection. The plants will use the ammonium at lower pH and the chloramine can kill all useful infusoria if treated with Amquel, Prime, etc. [Well, the cure kills the critters, not the chloramine.]

Wright