07-15-2011, 02:36 PM
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first thing i would do is grab a few test kits. right now ammonia and nitrite should do, but i would highly recommend a nitrate test too if you are concerned about the fishes overall health. these tanks are certainly more crowded then i do myself when i set up echo-system type aquariums, but i don't think they are at too critical of a level. the test will tell. another thing to keep in mind is sometimes old neglected tanks are actually better off then a lot of new tanks, so don't do anything too drastic with the cleaning, especially if tank levels are good.
that being said, the fish that concern me the most as far as adding to the tanks bioload are those plecos. if they are around 4" or over you want them gone. i would add them to the 40gal for now and decide later if you want to keep them, plecos will out grow all but the largest tanks at some point, and are very messy. as far as compatibility, at least all the fish are generally from the same waters and compatible, aside from the fact that angels will eat smaller tetras, neons and black, when they get larger. Bering that in mind, the angels may actually miss seeing a school of tetras swimming around the tank, they certainly wont be bothered by it unless breeding. you could consider adding the serpa tetras along with the plecos.
as for adapting them, you could do a drip method if you are concerned, but if its all the same tap water and temp, it shouldn't be much of a shock to their systems. this is assuming that the ph is generally the same, of course.