I am by no means an expert, but have been witnessing "occasional " flashing for some time, and did some recent research. the result was probably gill flukes. Flashing is a sign of gill irritation. It is usually a sign of parasites on the gills, but not 100% always. Ammonia burns can cause it, and from what I understand, permanent damage from ammonia burns may also cause it. The more common parasites that cause it are First ICH, second flukes and third is probably velvet. Ich so often hides and is so common in the hobby that I generrally go there first (Salt/ heat check out the article). Velvet is a fast worker, usually from what I've read a fish won't fend it off long if at all, and you will either identify and treat it, or lose fish quickly Treatment for velvet pretty harsh in reality, from what I've read Formalin and malechite green are the most successful, I am not personally fond of these but if I had velvet I imagine I'd use them. Velvet also can photosynthesize, and therefore a blackout in conjunction with treatment is a big help.
Flukes seem to be more common in tropical fish than I had ever realised, and they will hang around for ages, causing minor irritation for the most part. If they happen to infest a weak fish heavily, the end result usually shows up as a fungal or bacterial infection and often happens rapidly enough to make treatment efforts rough at best. The flukes themselves aren't deadly, but wekend damaged fish are quick to succomb to things like fungus and bacteria that won't normally bother them when thoir immune sytems are strong. The more crowded a tank is the more flukes will prosper. The crowded tank, fungus/bacteria thing was the key for me. I have had Guppies flashing occasionally for some time, and have had two in nine months which just suddenly got covered in white and died very rapidly. My guppy breeder tank at times gets crowded, and it did seem in retrospect that during crowded times there was more Flashing.
Prazi In preliminary experience is a blessing. Fluke tabs are effective but very harsh, prazi has no noticeable effect on my bio-filter, my snails, my plants, or even my fish. It is a bit of a bear to dissolve in water though. One treatment (one dose 7 days) is said to be sufficient, I did two because I hate suprises. I am currently into day 2 of week two, and am very certain my problem was flukes. After adding the prazi, the flashing quit completely within 24 hours. This is of course is hard to verify, because it was occasional, but there was seldom a week that I couldn't see at least one fish flash in any given day.
Prazi claims it will kill ich, but I am not sure. I do know it would have to be at least two and probably three back to back 7 day treatments to be totally effective with ich. I have seen it reccomended from LFs's for internal parasites, but honestly haven't seen anything on the labeling or on various website to indicate that it will work on internals. For this I would use Metrondazole, Or oin odd ball stuff I would do a lot of research.
Some additional info can be found here:
http://www.skepticalaquarist.com/docs/health/trematodes.shtml
Follow their links to Koivet and on to "Pond rx" if you need a source for Prazi. It is expensive, because it comes in large quantities ( by tropical tank standards) but I haven't price the prazi pro anywhere yet. So I may have paid too much.
dave