Hi and welcome to the forum
Sorry about the belated reply. Your thread is another one that has been hiding somewhere.
Serpae tetras are renown fin nippers and should not be kept with slow moving fishes or fish that have long flowing fins. Serpaes also come from soft acid water, wheras mollies come from hard alkaline water.
I would move the serpaes into the smaller tank and find out how hard the water is for the mollies. Livebearers like mollies, swordtails, platies & guppies need water with lots of minerals in, and a general hardness (GH) above 200ppm for guppies, swords & platies, and above 250ppm for mollies. If the water is too soft (low in minerals) the mollies will have problems.
You can buy GH test kits from pet shops or get the local pet shop to test a sample of tap water for you. You can also check with your water supply company to find out what the GH is. The GH only needs to be checked a few times each year so most people get their local pet shop to test the GH for them.
If you get the pet shop to test the water, write the results down (in numbers) at the time they do the test, then post the results here.
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If you want to buy any test kits, try to get liquid test kits rather than paper strip test kits because the liquid ones are more accurate, and check the expiry date. Try to buy kits from a cool area and avoid getting any test kits or medications that are kept in a warm fish room, in front of a window, or near any heat source.
Test kits you should get include: Ammonia (NH3/NH4), Nitrite (NO2), Nitrate (NO3) and pH.
General hardness (GH) and Carbonate hardness (KH) test kits are optional and not normally required on any regular basis. The other test kits mentioned are required when you first start an aquarium and if you have any problems with the fish.
All the test kits mentioned should be available from any pet shop or online.
When you have test kits at home, keep them cool and dry. I kept mine in a plastic icecream container with lid in the fridge.
*NB* Make sure children and animals can't get them because they contain poisonous chemicals. And wash your hands with soapy water after using them or working in the tank.
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If your tanks are newly set up (less than 2 months old), I would not add any more fish until they have cycled. Cycling a tank is where you allow beneficial filter bacteria to develop in the filter and this bacteria helps keep the water clean and safe for fish and other aquatic inhabitants (shrimp, snails, etc).
It takes about 4-5 weeks for a tropical aquarium to cycle and during that time you should monitor the water for ammonia, nitrite and pH. You need to keep the ammonia and nitrite levels as low as possible and keep the pH stabile.
Anything that breaks down in the water (fish food, fish waste, dead fish, rotting plant, etc) will produce ammonia. You get various bacteria that appear in the tank and they eat the ammonia and convert it into nitrite. After a few weeks more bacteria appear and convert the nitrite into nitrate. When the ammonia levels have gone up and come down to 0, and the nitrite has gone up and come down to 0, and the nitrates start going up, the tank is cycled and the filters have developed the beneficial bacteria needed to help keep the water clean.
During the cycling process you need to minimise feeding and do regular water changes to keep the ammonia and nitrite levels as low as possible. I usually recommend feeding a couple of times a week and doing a 75% water change and gravel cleaning the substrate 4-8 hours after feeding. You should also do a 75% water change and gravel clean the substrate on any day there is an ammonia or nitrite reading above 0. And do a 75% water change and gravel clean the substrate if the nitrates go above 20ppm.
Don't bother testing for nitrates while the tank is cycling because nitrate test kits will read nitrite as nitrate and give you a false reading.
Do check your tap water for ammonia, nitrite, nitrate & pH about once a month to make sure it is safe. In the UK and some other countries there is nitrate in the water and this can cause an issue to fish if the levels are too high.
You can get liquid bacterial supplements that help speed up the cycling process. These are best added at a double dose each day for a week then that should do it. Try to add the supplements near the filter intake so they get drawn into the filter where they are required.
If you can't get bacterial supplements, don't worry, tanks will cycle fine without them.
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Depending on what type of filter you have, you should not clean new filters for the first 6 weeks of life otherwise you can interfere with the filter bacteria establishing on the filter materials. Then once the filters have established, you should clean them at least once a month and preferably every 2 weeks.
If you have a power filter you should wash the filter materials in a bucket of tank water and then put them in the tank. You can wash the filter case and motor under tap water. Then put the filter materials back in the filter and set it back up.
If you have an undergravel filter (plastic plate that sits underneath the gravel and has an air pump blowing bubbles into it), these are cleaned with a basic model gravel cleaner when you do water changes and gravel clean the substrate.
If you don't have a gravel cleaner, get a basic model like the one in the following link.
https://www.about-goldfish.com/aquarium-cleaning.html
You should also get a couple of new buckets and use a permanent marker to write "FISH ONLY" on them. Keep the buckets with the fish keeping gear (gravel cleaner, nets, etc) and use them only for the fish.
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If your fish get sick or look funny, check the water and do water changes.
The following link is long and boring but is worth reading when you have some spare time. You can print it out and read in in bed to fall asleep.
http://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/
