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I had asked the same question and Rick was good enough to send me a picture outlining it but then pointed out it would be good share it here for the benefit of everyone.
In my opinion, almost always a bacterial issue at a very early age. In this case I beleive it happened while the fry was still on the cone and had yet to go free swimming. I was a bit befuddled, as I had three successful spawns prior to this one and one right afterwards. And I adhered to pretty much the same WC/breeder pair-prep regimen which was:
4 hour PP bath of the breeding pair 2-3 days prior to spawn (all my pairs were on a cycle so I could pretty much guess when they were goiing to lay)
60-70% water change of the breeding tank daily.
Dosed with meth blue 2-3 hours after eggs were laid
40-50% water change dialy while eggs were on the cone
40-50% water changes pre-free swimming
25% water change day of free swimming
50% water change after attachment
I live in a small city SE of Greater Grand Rapids (Caledonia). We have municipal water, but the water is still basically well water. There is virutally no chlorine and zero chloramine in my water. If this is a contributing factor I don't know, but it is something I felt worth mentioning.
I think the biggest issue was the parents themselves. While they were co-operative, and not egg/fry eaters, they were what I would call lazy parents. For instance, most of my pairs almost always move the fry to a different local on the cone after they have hatched. This pair felt content to let them sit and they became entangled in fungus from the non fertilzed eggs. The parents also did not appear to fan the eggs/wigglers nearly as much as my other pairs.
Maybe for some reason certain batches of fry are more prone to infection at an early age (underdeveloped immune system?) than others. At first I even considered the possibility of a genetic nature, but the effected areas seem to be too random for that.
What I find interesting is that you would think fry like this would be weak and sickly, but despite thier butt ugly appearence, they seem to be a very helathy batch. I don't know how to explain that, but after you have raised a few batches you start to get a "feel" for that by watching how they behave.
Would it be beneficial or helpful in any way to add an antibiotic after the eggs were laid?
Thats a tough question and a controversial one at that. While yes, it most likely would help, the long term danger is it wold stand a chance of helping to create drug resistant strain(s) of bacteria, and its hard to tell if it would have any residual effect (weak liver/kidneys etc) on the fry themselves. It is a good question for Al to chime in on.......
Would it be beneficial or helpful in any way to add an antibiotic after the eggs were laid?
There is a number of studies concening the use of chemicals in aquaculture and their uses including their residual effects. The link below is to a Greenpeace 'technical note': "Chemical Usage in Aquaculture: Implications for Residues in Market Products". You may find the tables on pages 6 - 9 a useful starting point for further studies? A variety of common chemicals commonly used in aquaculture are listed including antibiotics . . . (& Trichlorfon Rick)
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