Switching from gravel to plant soil without tearing down
The gravel was the original gravel, pale and about 4 mm, and after two years the root feeders were sulking. I did not want to break the tank down and re-cycle it, so I did the swap in place, one half of the floor at a time, with the fish still in. It took three Sundays and I would do it again, with one change.
Method: siphon out roughly 40 percent of the water into buckets, lift the plants from the left half and float them in one bucket, scoop out the old gravel with a plastic cup and a dustpan brush, and then lay 3 cm of planted-tank soil through a wide piece of pipe held against the glass so it did not cloud everything. I refilled by pouring the old tank water onto a saucer sitting on the new soil. The water went tea-colored for about a day and cleared by the next morning.
Plant soil leaches ammonia when it is new. On the first week I tested every evening: day one 0.25 mg/l, day two 0.5 mg/l, day three back to 0.25, zero by day five. I did a 25 percent water change on day two and day four, and the fish showed no stress that I could see. The second half, done two weeks later, barely moved the needle at all, probably because the filter had already had a taste of it.
The thing I would change is the order. I did the left side first because it was easier to reach, but that is where the filter intake is, and the fine dust got pulled straight into the sponge. The flow dropped to a trickle until I rinsed it in tank water on day three. Next time: far side first, intake side last, and a sock over the intake for the first 48 hours.
Six weeks on, the crypts have doubled, the sword plant has sent up two new leaves a week, and I have stopped dosing root tabs. The pH dropped from 7.6 to around 6.9 and has stayed there, which the tetras seem to prefer. The old gravel is in a bucket on the balcony, waiting for a purpose.