two betta tanks--only one has film

Question
Lynda,  We have two bettas in two small tanks (about 3/4 gal). We have twin sons, hence two of everything!  The living conditions of the fish are the same--same food, same water, same cleaning regime. We never allow excess food--we make sure they eat each pellet before giving them another.  Yet one of the tanks develops a grayish film while the other is crystal clear. So we know it's not something we're doing or not doing. The only differences are that they are different varieties of betta--can't imagine that makes a difference. But also the one tank is slightly smaller than the other.  We've been doing partial changes of water only.  With our last betta, which lived for at least two years, I took him out and gave his tank and everything in it a good cleaning with very hot water (no soap) every two weeks.  Perhaps this smaller tank is below some critical size that makes it difficult to avoid waste build-up?  You thoughts?

Answer
Hello,
First I would like to mention that keeping bettas in 3/4 gallon bowls is not right.  The bettas may have shallow water in his habitat, but he has lots of room to swim.  Information on the betta is not very easy to find, and vendors do not tell the truth when selling these poor fish.  They are there to make a buck, and this is all that matters to them.  
The film is simply that one betta is blowing more than the other one.  They should never see each other, so I am hoping you are making sure of them.  Bettas who constantly see each other get stressed, and sick.
Here is some information on the Betta, and how he should be kept.  Betta that are will kept will live up to 7 years.

Bettas should be kept in 5 gallon tanks, and there water should be around 78-80 degrees.  When bringing home a betta, we all know that the little sadistic cups they are kept in, are not heated, so we put the betta in unheated water, around the temperature he came in.  Then we heat his water slowly, very very slowly, ONE degree every two days as bringing up their temperature too fast could cause death.  Adding a teaspoon of undiluted aquarium salt to his water helps with stress, and parasites.  (One teaspoon to five gallons of water)  In his habitat the betta may live in shallow waters, but he has lots of room to swim.  Small tanks, bowls, vases, and mirrored tanks are all money making machines, and not suited for the betta.  Vendors do not care, as long as they sell.  They are there to make a buck, and this is all that matters to them.  Bettas kept in small containers will get sick, and their lives are cut short.  They are intelligent little beings, and should not be treated this way.  Unfortunately, their are no laws to protect fish, and not much good information on how the betta should be kept.
Bettas should have varied diets.  Betta Pellets, Betta Flakes, Bloodworms, Glassworms, Plankton Flakes, brine shrimp, and one frozen pea a week.  One day you feed him pellets the next flakes, and so on.  He must be fed in small quantities 2-3 times a day.  Example:  2 pellets for one meal, or 2 flakes for one meal.  Their tummy is the size of his eye, and we must always remember this when feeding them.  One cooked frozen pea a week is a must.  Cook the pea, remove the outer layer, and cut it into tiny pieces.  Feed the pieces one at a time to your betta making sure he eats it, and that it doesn't fall to the bottom of the tank.  On the day you feed him the pea, feed him nothing else.  This is so he cleans out his system, and avoids Swim Bladder Disease.  The Betta is prone to this disease which begins with constipation, so it is better to avoid it.
I hope this information helps, and if you have any other questions, please do not hesitate to write me.  
Lynda