Don't trust that LFS guy for any real information as he clearly is completely ignorant of or apathetic to the animals he sells. Perhaps both. There are many pet shops that will give you false information because either they don't know, or they don't care. In fact that sort of pet shops far outweighs the good shops, and that is regardless of the animal, but far more prevalent with fish.
The only reason that plant would pollute the water is if it was dead and decaying which it clearly isn't. Actually your plant is thriving. Unfortunately it's the only thing that is thriving, and will be the only thing living if you don't remedy the situation.
Your fish look like Ryukins too me rather than red-cap orandas, but their care is the same. They will need ten gallons of water each and a good filter, as well as a possible bubble curtain.
Here's the trouble.
1. Oxygen. Goldfish use a LOT of oxygen very quickly. While plants help a bit, alone they aren't enough to cover the needs of really any fish kept singly, let alone two goldfish. Most of your oxygen exchange happens at the surface of the water, so the more surface, the better (ie: a longer tank will be beter than a tall tank). Filter outputs and bubble curtains will both disturb the surface of the water and this movement aids in the exchange of oxygen by increasing the amount of surface area by means of the waves and currents they produce.
2. Room. Sure, those guys are small now, but they are going to get much bigger. Like I said before, they should reach 7-10 inches. Your LFS guy may know this, or he may not. Most goldfish don't make it to this size because they are put in poor conditions and they die, or are stunted and die, long before their natural time (not sure on fancy goldfish, but commons/comets have been known to live 20-40 years.
3. Water quality. Goldfish, and all fish, are constantly producing waste, much like humans. They excrete ammonia and urea, as well as being quite the little poop machines. This means they need a good filter and a cycled tank. A cycled tank refers to the nitrogen cycle where (in short) fish excrete ammonia (which is toxic to them) --- ammonia is consumed by bacteria and excreted as nitrite (which is also toxic to them, though slightly less so than ammonia) --- nitrite is consumed by bacteria and excreted as nitrate (which is also toxic but only at exceptionally high levels) --- nitrate is removed by the fishkeeper during the weekly partial water change/gravel vac. It is thoroughly important that you have a filter for this cycle to happen properly. The filter, besides removing the gunk (uneaten food, fish poo), is also the home of the good bacteria that is turning all that toxic natiness to semi-safe nitrate.
I'm not blaming you for not knowing, and neither is anyone else on this forum, but you seem very reluctant to listen to us and it does become a little frustrating. You aren't going to recieve different ideas on what these fish should be living in, at least, not so different that they conform to the set up you have. many people will recommend 20 gallons for the first fancy goldfish and ten for every fancy goldie after that. WE are only saying ten each. The simple truth is that all animals need to be researched before purchase, whether that animal is finned, feathered, furred, or otherwise, and sadly petstores are generally very bad places to go for that information.
