oxygen is produced by growing plants. that is a fact. under many cases. a decent amount of light and c02 can produce a nice balance between c02 and oxygen levels in the water.
to anyone that disagrees with surface agitation.. more power to you. if you agree with film. more power to you.
HOWEVER there are those of us that would prefer to keep plants under higher light levels to get certain colors and or growth rates and also would prefer not to have scum sitting on the top of the tank and have that removed by paper towel every couple of days
in a healthy planted tank with adequate lighting to grow just about any plant and its appropriate color a good amount of c02 will be injected into a tank this can stress fish and the biological filter.. the way around is increasing o2 levels. it does FAR more than relieve stress on fish, or increase pearling
plants need oxygen as well. bio filter needs it to breakdown about 80% of all organics in the tank to usable form by plants or other bacterium this also helps reduce algae as it feeds on ammonia and other organics that plants usually let bacteria make into easier form (plants still prefer ammonia to nitrate though)
wasted c02 is inconsequential to the benefits of added oxygen. there is no research you can provide me with that states otherwise.. my algae problem was due to lack of oxygen i had bba all over the place.. but my fish could barely breath and my plants weren't growing well
i add a sump it wastes c02 and increases oxygen and what do u know bba is showing signs of dissapearing. i've been down ferts roads. raising and lowering c02 levels. trying pumps and blah blah
efficient healthy bacteria and fish come from proper gas exchange.. my DC is still yellow but my ecosystem is far healthier. fish are far healthier and happier
surface film detracts from gas exchange.
low tech tank.. you don't want agitation? no problem. i have one. and when the fitler flow is reduce and the surface stops being broken well and turned over. film starts forming and small bubbles from c02 can be seen collecting i've added c02 and not added c02 on it.. it goes both ways.. the demand isn't high for either gas and it saved on c02 when i used it.. i don't even need it though as its low enough light platns grow fine without c02
its not as important in low tech but again the addition of oxygen is beneficial
as for film. it can be caused by poor filter flow and maintenance. or undersized filtration
it could be the lack of oxygen.. which i forgot to mention also changes redox potential. not as important for the water column but what is going on in the substrate? that plays a major roll in the health of an aquarium
i would suggest a good cleaning of filter media using tank water. siphon some of ur substrate lightly to removed some packed mulm.. don't over do it because it is beneficial to plants
add an airstone at night. it will break up the film and reduce organics in the tank as they will bond to rising bubbles. this will make the rim dirty. keep it clean so u don't have to scrub it off later
allowing the outflow of ur filter to disturb the surface is great for gas and protein exhcange as well. the best way to handle scum is TURNOVER.. u want the water at the top that is rich in oxygen to be pushed to the bottom. this also carries the film back into the water where it will get processed by the filter. it also helps keep the water aerated which we've already discussed as beneficial in anycase. EVEN low tech garners benefits. efficient conversion of ammonia is the best way to prevent algae. period
by the way OP ur plants do look good
and the bubbles will not hurt anything that way either