Hi - I think, unfortunately, you will probably need to return most of those fish to your LFS and rethink your stock choice as most of those fish are wrong for that size and shape tank.
A lot of staff working in LFS's will tell you anything in order to make a sale. It's always best to do your homework on a particular type of fish before buying. Or you can ask a few important questions in the shop and if they are unsure of the answer, ask them to look it up in one of their books (most of them will have a few tropical fish books on the shelves!).
You need to be asking what the adult size of the fish will be. That way you can avoid wasting money buying fish that will outgrow your tank and need rehoming, which might be difficult. Next find out whether they are shoaling fish or can be kept singly or in pairs etc. Also check on what type of water conditions and substrate they prefer, or whether they are cave dwellers etc. Each species is different and by checking out these things beforehand can make a lot of difference to the well-being of the fish. A happy fish is a healthy fish!
Next you want to find out what that particular fish is compatible with. In your tank you have an Angel fish and you added guppies. Not a good combination as Angels are terrible fin-nippers and guppies will end up with shredded fins and possibly die from stress.
If you can, try to get a mix of top, middle and bottom dwellers - but you won't be able to do this very well in a small tank with shoaling fish species as you are limited to the amount of fish you can stock. Look for dwarf varieties, perhaps.
If the person in your LFS is being unhelpful, make a note of the fish you are interested in, then you can always post questions on here for advice before making a purchase - or just Google and see what you find that way.
It might seem a bit long-winded but it's the best thing to do to avoid a situation like the one you're in at the moment.
Also, another bit of advice would be to invest in a liquid testing kit as the paper strip tests are very unreliable.
Remember to do your weekly 25% water changes, too. That will keep ammonia levels down, especially a few days/week after adding any new fish as the more fish you have the higher the ammonia/nitrites levels.
As a rough estimate they say you can keep an inch of fish per gallon. If yours is about a US 12 gallon that doesn't mean 12 fish of any species or size - you have to find out the adult size of each fish you buy and work it out from that. So, let's say you end up stocking the tank with fish that don't grow larger than 2" each - then you can stock with about 6 fish, even a few more if your tank conditions are kept tip-top. But if you put in a fish that grows to 6", then it will mean less of the smaller fish.
A slightly understocked tank is always easier for beginners to manage than an over-stocked tank that needs a lot of high maintenance.
Hope I've not bombarded you with too much advice in one go
All the best - Athena
and we only find the best way to do it through experience and getting advice online, like on here.
