ok dawn,i need you !! lol AGAIN !!

the giant danios and the 3 angels in the 20g at "ADULT" growth will be too much for the tank imo.
What size are we...I mean you..upgrading to?

I wouldn't suggest 1 cory, they are a social species and do soo much better in groups of a min. 3-4 (but even more are better from what I've experienced).

IF your lucky enough to have a lfs or friend in need and you really want the cory I'd rehome the danios and go with a small school of the corydoras!
I had the giant danios, great dither fish, but not much to look at ime.

Is this a 20g Long/ High/etc?
20 high 24x12x16
20 long 30x12x12
20 tall 20x10x24
20 hexagon 18x16x20

 

I 2nd Dawn's advice 200%!

 

if you leave them all in the 20g long term you risk stunting their growth.

Next they are a social fish, unless you end up with a mated pair..then things can get nasty for the outsider ime.
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55g would be perferct for a pair (again, with the outsider theory...hence the runt of the group)...Recommends vals for them when you upgrade to the 55g

When you upgrade to the 55g get yourself 4-5 corydoras for the bottom

Next, if you'd like to add a peaceful pleco to your 55g you could do ..yes, here comes my favorite! *drum roll* A ...LONGFIN bristlenose pleco (albinos are pretty)

And for your top levels of water you could do 4 or 5 Hatchetfish

 

<a class="postlink" href="http://www.angelfishusa.net/shopcontent.asp?type=pinoyblue" onclick="window.open(this.href);return false;">http://www.angelfishusa.net/shopcontent ... =pinoyblue</a>

If your stocking a couple go OVERKILL on the filtration (something like an xp4 and possibly ac110 as hob) for the 55g -->will be money well spent ime.

I wouldn't go over 4-5 in a 55g if your looking for a "number", but that's only with really good filtration. You'll want them to have plenty of room to achieve full growth and allow some room in case you luck out and get fry

link to vals--> <a class="postlink" href="http://www.aquariumplants.com/Vallisneria_s/34.htm" onclick="window.open(this.href);return false;">http://www.aquariumplants.com/Vallisneria_s/34.htm</a>
Angels like to swim through them

 

when you get the larger tank toss them (plants) over into it....You could always use your current tank as either a sump or a fry tank...unless you plan on stocking it as a 2nd tank?

 

It does!

Yes... OVERFILTERING is ALWAYS best! 4 Angels will be good in a 55... heck.. 6 would be ok with a good filtration system UNTIL they go to pair up... then there will be some territorial issues, but that goes for any tank.

When my Angels first paired up they were in my 125 with my Discus and they marked 1/2 the tank as their territory! So, regardless of tank size they get VERY territorial when spawning and wont let anything near them... that is when you either need to separate them or move them both to another tank to breed depending on what you want to do. Keep in mind it requires several tanks if you wish to breed them and have any live to sale size (dime to quarter size)

 

Weekly water changes is good!

Also that is a good filter... but NOT for a 55 gallon tank! 2-3 of those would work for a 55 but its better to get the Rena XP4 that Dawn recommended for the 55 and maybe an aquaclear 110 (or whatever the largest one is)

 

No problem... I used to and still do love observing Angelfish behavior... unfortunately I dont find them to be as attractive as some of the other South American fish out there!

Needless to say, I still kept several Angelfish with what I would consider success as I did not loose any and managed to get a REALLY good pair out of my time spent with them.

Also Angels and Discus have VERY similar needs...

Another thing to keep in mind, the reason Dawn recommended 2 filters is when you maintain the filters and clean them you alternate back and forth so you dont loose beneficial bacteria when you clean (ie: 1 week clean the canister, 2 weeks later clean the HOB, then repeat)

Also in addition to 2 filters having 2 or 3 heaters (about 200 watts) in the 55 is a good idea in case one goes out. It also helps because setting the temp at the higher temp that Angels thrive in will keep you from needing to keep 1 300w heater cranked all the way up! I have 4 heaters in my 125, 2 in my 55 and 2 in my 30... I do that for that reason... and YES I have lost quite a few heaters so its no big deal when one goes out (as long as it does not get stuck on all the time) Always a good idea to keep a backup!

 

oh, the pinkish green plant is the one I sent him I bet>>thats ludwigia repens>>Moderate-High Light plant.

Anytime you see ones other than green you can pretty much bank the paycheck that your going to need moderate to high lighting!

I'd look over aquariumplants.com to get some ideas and then I can help you once you tell me your lighting type and wpg over the tank