Alright! Congrats and welcome to the hobby. It's addicting and gets even worse when you take the leap into planted tanks. That's where I started.
You have two female guppies. Bad news: females are practically born pregnant. Nearly every female from the LFS will join your tank full of babies.
Worse news: they don't even have to breed every time they drop a clutch!
Good news: there isn't much room to hide in your tank. So babies will be exposed.
Better news: neons and guppies LOVE to eat fry. I've seen my guppies pop out fry and turn around and eat them.
I admire that you didn't do the trendy thing and buy males. I don't have an answer for your comet issue. If you can return him do it. Other wise.... Sigh. Water changes will need to be a mandatory weekly thing, maybe biweekly depending on your water readings. As you're cycling it's going to be a delicate thing.
Plus 1 for prime! Prime is your best friend.
After the tank runs for several months I say more cories! Preferably the same markings as the ones you already have. I have 6 in my tank 3 like you're, 2 peppered, 1 albino and they pretty much stuck with those who look like them, they'll mingle some times but I rarely see the peppered apart and the false juli stay pretty close too.
As for a center piece I bet a dwarf gourami would be okay.
Once you get the hang of this, come to the dark side and learn about planted aquariums. Good luck.
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What is plus 1 for prime for?? I try to google but couldn't find anything. I have API quick start and Prime for taking away chlorine from water ... Wait!!! Are we talking about the same prime??????
And yes a dwarf would probably be the centerpiece ... I'm order my water test kit and it should be here by the 12 or before that date ... When it comes I'll check the water.
I am adding fish weekly not too much just to see if they going to get along or not.
They told me to change the water monthly!? And I keep people saying to change water weekly or every 2 weeks.