Zebra danios, while small, are extremely active fish and most people wouldn't put them in a 20 gallon long.
Angels need more room and can be territorial. They are also slow moving and may get heir fins nipped by the mollies.
Guppies and swordtails are ok, from what I know.
Mollies also are larger and need more space.
You will for sure ruin to problems with the GCAE.
All of those fish in a 12 g cycled is a problem, not cycled and you are guaranteed to see a crash soon.
I recommend rehoming all of them and starting a cycle.
Anubias, crypt., Java fern
Giger, blue male crown tail betta
Sorry to say but you have a lot of things you must address if you want a healthy tank.
First off, considering you've had this tank for a week, you almost certainly haven't cycled it. In all mature tanks, colonies of beneficial bactia convert ammonia (extremely toxic) from fish waste into nitrite and then nitrate (much less toxic), which can be removed with weekly water changes. This allows fish to be kept in confined spaces in the first place. Accumulating these bacteria is called the cycle, and I highly recommend you check out the fish-in cycle instructions in the beginner's section. Knowing your water parameters is very important, and I recommend buying a liquid test kit for ammonia, nitrite, nitrate and pH as soon as possible.
Your stocking also isn't ideal, and many (most?) of your fish will need to be r-homed or returned, if possible. Only the guppies will do well in 12 gallons of water. Zebra danios are too active for a 12 gallon. Both mollies and swordtails get to 4 inches or more and can be aggressive. The Chinese algae eaters will certainly exceed 4 inches and with good care may also 6. As mentioned, they also are territorial. Angels will grow to 4-6 inches long and just as the algae eaters will chase and fight other fish.
As Talldutchie mentioned, the crushed seashell in your tank will leach calcium carbonate and make your water very hard and alkaline. This might suit the livebearers, but not so much the danios nor the angels nor any soft water community fish you eventually decide to get.
Last but not least, in the future, please research everything you do in this hobby (although this piece of advice also applies well elsewhere in life). Fishkeeping is much more complicated than most people think, and uninformed decisions can lead to disaster.
Why don't pet stores ask how much experience people have, and teach the new guys from the very beginning! All the above advice is great advice. Definitely check out the beginners corner of the forum. Lots of helpful tips and guides there! If you don't mind me asking, to buy all these fish and the aquarium it's self, did you go to a local store, Petsmart, or Petco? Who ever sold you everything should've known better. Unless everything was bought separately, you were working with someone who just wanted to make a sale...
Planted 37 gallon, Low tech, with Cherry Barbs, Cardinal tetras, and red cherry shrimp.
All have given you great advise. Please reread, in particular, Madagascariensis's post. He tells you all you need to know.
Please don't hesitate to ask more questions. We want you to have a tank you'll enjoy and fish that will be healthy. Right now, you're on a short road to disaster :o(
30 g FW planted:corys, ABNP, blue angel, harleys, zebra danios, apple & nerite snails
15 g FW planted:crown tail betta, neons, pygmy cory, clown pleco,apple & nerite snails
90 g FW planted:congos, rainbows, roseline sharks, swordtails, ottos,krib pair, ABNP, peppered cories, apple snails
90 Gal Journal: http://bit.ly/1vC7gVX
fishless cycling: http://bit.ly/1DARf3T
fish in cycling: http://bit.ly/1ILvcfp
Great advice from the above members...
I'll also add... what type of light do you have over the tank? If the bulb is tinted blue it is probably acintic (for salt water) and will cause future algae problems. Best flourescents are a daylight or a full spectrum bulb
Thanks for all the replies you guys!
In all honesty, I just got the aquarium very impulsively so I didn't do any proper research, which I almost always do.
I'm from Bangladesh and there aren't franchise pet stores here. I went to a local store, from where I got the aquarium and most of the fish. The people working in these stores are no help at all.
I've had Angels and Sword tails in the past and they seemed to be good community fish. However, I noticed one of the Angels being aggressive with the guppies, Sword Tails and the other Angel. One of the zebra fish also seems to be harassing the other zebra fish, guppies and the sword tails. The male black molly also seems to be harassing the female.
It's beyond evident that I've made some very poor choices when picking out the fish. I still don't want to re home all of 'em though.
I'll check out the beginners section this weekend. (swamped at work atm)
As for the lighting, I have a blue tube light. My tank gets a little bit sunlight during the day, 'cause I have a lot of real plants in the tank, but nothing direct though. And I don't keep the blue light on for more than a couple of hours at night.
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