I am so sorry to hear you are having problems with your fish.
The first thing I suggest is to get rid of the strip tests and purchase a liquid test kit as they are much more accurate in reading the water parameters. API Master kit is a good one if you can get it. If you cannot find one locally you may want to order one online from somewhere like
http://www.drsfostersmith.com/product/prod_display.cfm?c=3578+4345+4454&pcatid=4454Also if you go to the store to have a water test run ask them for specific numbers so you know what the test results really are.... to one person "good" can mean a totally different thing than to another.
If you are showing ammonia and nitrite readings your tank has lost it's cycle and you need to continue water changes daily to keep those numbers under control. High nitrates can also cause the veining in the tails of your fish. Again water changes will keep this number down. Once the tank is fully cycled again I would suggest doing at least a 25% water change EVERY week not once a month. Also to help get your cycle started I suggest using Stability by Seachem. It is a bacterial supplement that will assist you in re-establishing balance in your tank.
A recommendation for the future... always quarantine new fish and plants before adding them to your tank as they can be carrying parasites/diseases that can affect your existing fish/plants.
At one point you say you can see white spots on the fish... do they appear like salt crystals?? If they do then it sounds like ICH. You need to vacuum the gravel to get that parasite out of your tank. ICH can be killed with Aquarium Salt in its free swimming stage (before you see it on the fish) but it is constantly hatching from eggs in the substrate so it is important to vacuum them out. See the this article for Salt treatment for illness -->
http://www.aquapuppies.com/aquapuppies/index.php/topic,5.0.htmlA second filter will never hurt your tank... I am not familiar with the particular filter you have mentioned and don't know what the Gallons Per Hour (gph) turnover it has so before buying a second one please look to see what the gph is and then figure out how much more gph you need to add to the original filter to achieve a 10 times your tank volume gph.
Are you dosing daily with the Melafix and Primafix? You should be if you are not. You can use these in conjunction with the salt with no adverse reactions. Still leaving the carbon out of your filter.
That is all I can think of for now for advice..... I will re-read your post and see if I can see anything I have neglected. Also someone else may have other good advice for you as well.
Best of luck with your fish!