How strong current tires an ageing school
Young tetras ride current for exercise; old tetras pay for it. A senior fish's muscle and stamina fade, so a flow a younger school barely notices becomes a treadmill the oldest fish can never step off. Watch for the fish pinned at the downstream end, hovering in the one slack pocket behind a rock, or resting on the substrate between efforts — they are showing you exactly where the water is too strong. A schooling species will also drift apart when the weakest can no longer hold formation, and a school that has stopped moving as one is often a flow problem wearing the disguise of old age.
Slowing the flow without losing filtration
You still need filtration; you need its delivery softened. Angle the filter outlet toward the nearest glass so the jet spends itself against the pane instead of crossing the tank, or fit a spray bar along the back wall so the return spreads into many small streams. A simple baffle — a piece of filter sponge fixed over the outlet — breaks a single jet into a gentle wash. If the filter has an adjustable flow rate, turn it down a step and watch the fish for a few days; if debris starts settling in corners, you have gone too far and can nudge it back up. Aim for water that moves everywhere but pushes nowhere.
Planting that shelters resting tetras
Plants are the furniture of slow water. Tall stem plants along the back and sides absorb current as it passes through them, and dense clumps create the still pockets older tetras need. Broad-leaved plants give a tired fish somewhere to hover in shade with minimal effort, and a thicket toward the downstream end catches fish that drift rather than swim. Plant more heavily than you would for a young school, and accept a slightly wilder look: for senior tetras, a jungle is comfort and an open, minimalist scape is exposure. Let some plants reach the surface so the calm zone extends through the whole water column, not just along the bottom. A few floating plants earn their place as well, dimming the top layer where the school rises to feed and softening the light the oldest eyes have to work under.
Arranging the tank around the slowest fish
Judge every change by the oldest tetra, not the average one. Food should land where the slow fish can reach it before the school strips it — spread flake or fine food across the surface rather than dropping it in one rich spot the strong fish monopolise. Keep lighting gentle at the edges so resting fish are not lit like a stage, and avoid rearranging hardscape often; familiar shelter is worth more to an old fish than a refreshed layout. Check in the evening, when the school settles, that every fish has found a pocket it can hold without effort.
Each adjustment lands better when the slowest fish sets the pace. For a second view on baffles or planting density, put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.