Well, I think the best you can do tonight is hope for the best. However, as a fairly new tank, yours many not be fully cycled. The link someone posted has good info about cycling a tank.
Probably either ammonia, or nitrites, or both are elevated enough to be causing problems. With my new tank I experienced this, being a newbie myself at the time.
Tonight, as an emergency treatment, just in case either chlorine or nitrites are the problem, you could do another partial water change, leaving them in the tank, and making sure the water temp is the same as the tank temperature. This would help to reduce harmful levels of ammonia and nitrite.
Also, I was helped by KarlTH, who is an expert in the chemistry of the aquarium, on one occasion when I had nitrite poisoning. The same treatment works for chlorine poisoning.
0.5 teaspoons table salt per 10 gallons. That will prevent uptake of the nitrite until you can do a water change.
This is like giving oxygen to a sick or injured person. The salt interferes with the action of the nitrites in the gills. The salt binds with the hemoglobin and allows the o2 to be carried to the body.
The hemoglobin in the blood is what carries oxygen to the cells, and nitrites bind with the hemoglobin and prevent the o2 from binding with the hemoglobin so the fish in effect suffocates.
The salt helps in a similar way with chlorine poisoning, in that the fish can breath better and get more o2.
Be sure to dilute the salt in a cup of tank water and gradually pour it into the tank.
I would do that after a water change tonight if you can. If you dose the salt tonight, and then do a water change in the morning, you need to replace the salinity that you removed.
Example: if you dose .5 tsp for 10 gallons, then tomorrow do a 50% water change, add .25 tsp to the water you're adding back.
You need to get the API Master liquid test kit. Strips aren't very accurate, and over a few weeks and months you really save money with the API kit using test tubes and drops of chemicals rather than strips.
It is imperative that you get testing equipment, to monitor ammonia, nitrites, nitrates, ph.
As soon as you can, tomorrow, get a product called
PRIME.
It's a great conditioner, and it detoxifies ammonia and nitrites for at least 24 hours and can really save your life, the life of your fishies that is, until you can do water changes to bring levels to safe limits.
I've read many many posts on this forum, and the consensus is that PRIME is a life saver.
We've all been there, with a new tank, and until your beneficial bacteria that eat ammonia, and then nitrite eating bacteria develope in your tank, you are going to have unstable water parameters.
Here's a great link to a thread that follows the progress of one of our members. When you have time, look through it. You'll learn a lot. Really, there are so many extremely experienced fishkeepers here, that want to help. You have come to the right place.
http://www.aquariacentral.com/forums/showthread.php?t=177542&highlight=katuuz
As you look for help on the AC you might want to start new threads in the 'Newbie" forum. I always titled it something like 'HELP HELP, nitrites!!" or something that helps convey your level of distress. It's not that people won't respond quickly, it's just that there are so many people looking for assistance that I guess the folks here sort of "triage" and go for the ones that seem the most serious first.
This is, I think, the best forum on the web for fishkeepers of all levels of experience. These people here really want to help, and you'll really learn a lot.
Welcome to the AC, and I hope sincerely, that your molly and the others will be OK.