my tetra seems paralysed

Question
QUESTION: Hi. I have 4 red-eye tetras in my 10 gallon fish tank (and 2 mollies and 1 platy). I got the tank only 1.5 weeks ago and am a beginner.
One of my tetras seems paralyzed. It was sitting at the bottom of the tank three days ago and stopped feeding. I thought it might be dying so, I prepared some water in a bucket at moved the fish there, but it didn't die the whole day, so I put it back in the tank.
It hasn't eaten for past 3 days now and either floats just near the surface alone or hides at the bottom of the tank. If other tetras pick on it, it swims around a little but very weirdly (not like swimming horizontal) but swims vertical and side ways. It looks paralyzed. Whats wrong with it?

ANSWER: Hello Abhishek,
Well, for starters, you don't want to move around the fish so much - that is just as stressful as whatever is causing it's illness in the first place. That's part of the reason why gravel vacuums are by far the best way to do your water changes - no moving around the fish, ever! Though it is a good idea to have an actual tank at the ready - set up and running with no fish for use as either a sick tank or a quarantine tank.
Now, seven fish are honestly too many for a tank that is so young. You will want to stick to adding no more than 1-2 fish per week, but right now you are at your maximum and don't want to add any other fish. But at least next time now you'll know.
As far as your paralyzed fish, there is something that is stressing it out, and it may very well begin to stress out your other fish soon also.
The first thing I would do - half tank water change. If you don't already have a gravel vacuum, get one (one that is the appropriate size for your tank) and use that from now on to do all your water changes. You will want to do a large one straight away - 50%, then gradually work to doing about 3 gallons once a week. You will also need a bucket of some sort that is used strictly for this purpose - and never put any chemicals into it.
Secondly, you should start testing your water a few times a week. You want the ammonia to read 0, as well as the nitrites, and the nitrates should never go higher than 20.
Chances are- your ammonia is too high, because your tank has not had even close to enough time to cycle before adding all these fish. What keeps a good tank going for a long time with healthy fish is your biofilter. This is the good bacteria that gradually grows over time as you add more and more fish. But if there are too many fish added at once, creating a whole bunch of ammonia and there is not enough good bacteria to eat it, then you will throw your biofilter off - your tank will have to start over in it's cycling process, and in the meantime you will have to deal with frequent water changes - two or three a week until the ammonia starts going down on it's own.
You can learn much much more by doing some more research about the Nitrogen cycles online and by reading books on freshwater fish care.
But this is just the basics to get you started. So don't worry - and don't give up on fish keeping. There is much to learn and you've only just begun, but we're here to help you on your journey to becoming your own fish expert :) Good luck - and start changing that water!
(Basically, for now, do small, frequent water changes using your gravel vacuum - you always want to take water from the bottom of your tank - and get some test strips to keep an eye on your a/n/n levels.)

---------- FOLLOW-UP ----------

QUESTION: Hi. Thanks for your answer and useful advice. I have been reading a lot in the past week on fish care.

This is not exactly a follow up but let me ask since I am a beginner. The mollies seem to be doing something strange.
Firstly, they lightly touch the objects in the aquarium and suddenly jump as if to scratch. But it really like a quick flashing movement and very fast.
Secondly, they have been coming to the surface of the tank and splashing water every now and then. I dont know what these two things mean. Can you please quide me with it? Thank you very much.

Answer
They are doing exactly what you said - flashing. That's the term we use when fish are trying to scratch themselves on the rocks/decor or whatever else. The most common cause is Parasites. These can be brought in by plants and other fish, and can even grow in poor water. Buy an all remedy that is gentle on fish that may or may not be sick - such as Lifeguard. That's especially a great one to use for beginner's because you don't have to worry so much about doses - it comes in a pouch that you just drop into the water, which will allow you to continue to do water changes as you need without constantly remeasuring the dosages for medication. It's very important to always follow the directions exactly,  because overdosing can be fatal to your fish, and/or kill of your biofilter, and under dosing won't do your fish any good. Let me know if you need anymore help. :)