Lots to deal with, so here goes. Major issue first.
"Cycling" refers to the establishment of the nitrifying bacteria that is essential in any fish tank. You can read about this here: http://www.tropicalfishkeeping.com/f...m-cycle-38617/
Ammonia and nitrite will rise during the cycling period, and these are highly toxic to fish. Daily partial water changes of half the tank is essential but at this stage it may or may not help them. Using a conditioner that detoxifies ammonia and nitrite is also helpful during cycling. Seachem's Prime does this, as does Aquarium Solutions' Ultimate.
I'm not trying to scare you, but be prepared for all these fish to die. Mollies are especially sensitive to ammonia, and they cannot survive levels much above zero. Sooner or later they will succumb.
Changing the gravel to sand would have removed some of the bacteria, but under the circumstances this has made little difference since the initial cycling is still the big issue.
I can't see the white spots on the fish in the photo, but this may be ich or white spot parasite. Ich occurs due to stress, and the considerable stress of new fish frequently causes it. Then there is the stress from the cycling issues making things worse. I'm reluctant to suggest treatment for ich, as it will add further to the stress and I do honestly think the fish will die whatever. But since you have livebearers that can tolerate salt, using salt is one way to deal with ich, plus raising the temperature to around 85F for a week.
Looking ahead to when all the above is over, to the plants. They need overhead light to grow, and nutrients (fertilizer). You initially said you feed the plants, but then you said you don't use fertilizer...what are you using? I'm going to leave this for now, simply because the above issues have to be resolved and I don't want to bog this down further.
Last comment, on the fish. A 10g tank is insufficient space for mollies, have a read of the profile for further info, click on Common Molly. Do not add a Chinese algae eater, these are more trouble than they're worth, read that profile. Algae is the very least of your problems. We can talk about suitable fish when this is settled.
There is some basic science in a successful aquarium, and it is unfortunate that the majority of stores never convey this to customers. Most of us have learned this the hard way, as you are now, so try not to be discouraged. Success is possible with the knowledge.