Fishing

Also on both lake and river my float after casting was at the correct height then 30 seconds later would very gradually sink down

On the river this is probably caused by the current pushing your float to the side and your sweetcorn catching on the bottom, slowly pulling the float under.

If you have a waggler set up (line only going through bottom of float) on the lake then it could be wind pushing the slack line (between rod and float) which pulls the float under. Your sweet corn will be sat on the bottom of the lake anchoring everything in place.

Sweet corn sinks so you have to take this into account when adding your shot and setting the depth. Sometimes the weight of the corn is enough to sink the float. On a lake you can normally get away with being a little over-depth as the corn will just sit there but on a river you need to be a bit under-depth to stop it from catching on the river bed as it is carried along by the current.

The fish you caught was a roach btw.