What the film is and why it forms
Most aquarium surface film is a biofilm: a thin mat of bacteria feeding on dissolved proteins and fats that float and collect wherever the surface is still. Its ingredients are ordinary tank life concentrated at the top — oils from rich foods, residue from fingers and hand creams dipped in during maintenance, fats released by frozen foods as they thaw, and the breakdown products of dying leaves. A light white or grey film is mostly unsightly, but it is not harmless decoration: a sealed surface slows the exchange of gases, so oxygen enters and carbon dioxide leaves less freely, and in a heavily stocked or warm tank that margin matters. A rainbow, petrol-like sheen is a different warning — it suggests an actual contaminant such as a spray used near the tank — and should be treated as a water-quality problem, not a cosmetic one.
Break it up with surface movement
Film can only exist on still water, so agitation is the permanent answer. Raise or angle the filter outlet so the return current visibly ripples and folds the surface; on a hang-on-back filter, keeping the water level high enough that the outflow skims across the top rather than dropping straight down makes a surprising difference. An airstone achieves the same from below, its bubble stream bursting the film at one point and setting the whole surface slowly circulating. Lily pipes and spray bars can be tilted a few degrees upward for the same effect. Once the surface is genuinely moving, film cannot re-form into a continuous sheet, and gas exchange improves as a bonus.
Lift it off and cut its food supply
For a film that is already established, lay a sheet of plain, undyed paper towel flat on the surface, let it wet through for a few seconds, then lift it away — the film comes with it. Repeat with fresh sheets until the surface looks clean. A dedicated surface skimmer, either a small powered unit or an intake attachment, does this job continuously and suits tanks whose layout leaves unavoidable dead-still corners. Then reduce what feeds the film: feed smaller amounts so less oil-rich food dissolves uneaten, rinse frozen foods in a small net before they go in if they cloud the water heavily, wash hands and rinse them plain before tank work, and remove melting leaves instead of letting them rot at the surface line.
When film points at a bigger problem
A film that rebuilds within hours of clearing, thicker and greyer each time, is reporting an organic overload — something decaying in the tank, a filter overdue for maintenance, or feeding well beyond the stock. Check for a dead fish or a rotting plant crown, test the water, and clean the filter media in tank water if flow has faded. Fish spending their mornings at the surface in a filmed-over tank are short of oxygen, not admiring the view, and need aeration and a partial water change that day. A true rainbow sheen after any chemical has been used in the room calls for larger water changes and fresh activated carbon in the filter while you find the source.
Keeping the surface open for good
The settled routine is simple: surface movement built into the filter setup, modest feeding, hands rinsed before they go in, and a paper-towel lift on water-change day if a faint film has gathered in a still corner. Tanks with floating plants will always hold small still patches beneath the leaves, which is normal and easily managed by keeping the plant cover partial rather than wall to wall. For a sheen you cannot place, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.