Hi dat1, welcome to the forums.
You can use lead fishing weights to get your driftwood to sink, tie them on carefully, I seem to remember somewhere that something got tangled in nylon that was used, and koura are nosey things.
When you say replace the sponge filter, are you just rinsing it out and putting it back, or chucking it out and putting a new sponge in there? The latter is not necessary, as the sponge does not deteriorate and can be used for years, and by throwing it away you are loosing beneficial bacteria from the filter.
If the filter contains ceramic media, just rinse this out when it gets clogged with gunk, best done in a bucket of water that you have removed from the tank so as not to kill off the bacteria, I only do mine when the flow reduces and it is in a net bag, so is very easy to haul out and give it a swish around in the bucket and pop it back in after I have done the same with the sponge, which usually gets a few squeezes in the bucket to clear the gunk as well.
HTH