Fix the frame before anything else
A diary only works if the photographs can be compared, so choose one viewpoint, straight-on from your usual viewing chair height, and shoot from it every single time. Mark the spot if it helps, and hold the phone at the same distance so the aquascape fills the frame identically week to week. Take the picture at the same point in the light cycle, with room lights off and curtains drawn, because daylight leaking in changes colours and invites reflections that fake differences between shots. A consistent frame makes a centimetre of carpet growth or a thinning patch of stems obvious instead of arguable.
Shoot the details as well as the whole
Alongside the full-tank shot, take two or three close frames of the areas you are watching: a foreground corner where algae starts, a slow plant you are willing back to health, the hardscape joint where mulm collects. Close shots catch what the wide view hides, such as the first thread of hair algae on a rock or new leaves unfurling on a plant that looked static for weeks. Keep the same close viewpoints too, so each area builds its own little sequence. When fish behaviour is part of the story, a short note beside the photo carries what a still image cannot.
Write the note while you take the picture
Every entry needs its annotation made on the spot: what you trimmed, fertilised, planted, moved or cleaned since the previous photograph, and what you are hoping to see by the next one. Without that note, a diary becomes a folder of pretty pictures that cannot explain themselves; with it, you can look back and connect cause to effect, the week the light hours went up and the algae answered, the trim that finally let the background stems thicken. Record water test results in the same entry when you take them, because plant pearling, stunting or melting often makes sense only next to the numbers from that day.
Review in runs, not single shots
The diary pays off when you scroll it as a sequence rather than admiring individual frames. Run through a few months quickly and the aquascape's direction of travel appears: whether the layout is maturing toward your picture of it or drifting, which plants are winning their space and which are quietly losing. That review is the moment to plan the next rescape touches, a hardscape shift, a plant swap, a lighting change, with evidence instead of a vague feeling that something is off. Archived milestone shots, planting day, first fill, the mature layout, become the before-and-after record you will wish you had for every tank.
Keep the diary with the tank's records
Store the sequence in the same place as the tank's other documentation rather than in a general camera roll, where it will be buried under everything else within a month. Filed in the tank's KinsLe Pet Passport, the dated photographs sit beside the maintenance notes and test logs that explain them, so the whole visual history of the aquascape travels with the tank's records if it is ever handed on.