I would be inclined to suspect that the cloudiness had nothing to do with the angel weakening and dying, but this is not by any means absolute. I will no longer acquire fish from "chain" stores, I have lost so many of them within say a year of acquisition (when fish from elsewhere are fine for years). Aside from this, the angel may have been injured during capture/transport. Was it acclimated slowly?
To the haze, I suspect this is a bacterial bloom, or possibly due to the Purigen product; I've never used this, but some of these chemical products can affect bacteria initially. I've no idea beyond the suggestion.
A bacterial bloom is most common in new tanks as Akasha said. Established tanks rarely have them, except when the balance is disturbed [explained below]. Heterotrophs appear sooner and faster. They build many of the biofilms that all bacteria use to adhere to surfaces, and they reproduce much faster, around 15 to 60 minutes, compared to hours for the autotrophs. Heterotrophs are facultative anaerobes—unlike autotrophs which need oxygen—so they can switch between aerobic and anaerobic depending upon the environment.
When heterotrophs bloom in the water they switch to being aerobic and consume vast amounts of oxygen. This is the real danger of a bacteria bloom, as it can starve the fish of oxygen. Increasing aeration may be advisable, but before jumping the gun I would carefully observe the fish. It is fairly easy to see increased respiration which is the first sign (more rapid gill movement), then nearing the surface more (actually at the surface in extreme cases), and usually lethargy. Bacterial blooms can come, last several days, and then suddenly clear with no harm to fish or bacteria, so I would not take remedial action unless the fish are indicating it needs it.
In an established tank, a bacterial bloom is caused by something that upsets the biological balance, normally by increasing the organic matter too quickly, such as overfeeding, excessive decaying plant and animal matter, excess waste from overcrowding, etc., but other imbalances can occur too. Here, the heterotrophs quickly reproduce by feeding on this organic matter. This produces ammonia as a by-product, and the sudden surge in ammonia overtakes the nitrifying bacteria that need time to "catch up." Live plants (especially floating) help here, as they can assimilate and/or take up considerable quantities of ammonia faster. Note that the bacterial bloom causes the rise in ammonia, not the opposite as we somes read. And here again, don't assume ammonia (test for it if you can, just to be certain) unless you see obvious signs, similar to the others above.
I sometimes see mild bacterial blooms after a water change, though this can also be due to microscopic sediment in the water. I usually see it very slightly after each filter clean.
Byron.
/www.fishforums.net/index.php?/topic/437410-help-angelfish-acting-weird/
