Ideally you want a 15 to 20 times flow rate in your tank. So for 15x the flow rate it would be 30x15=450 GPH, and with 20 times the flow rate it would be 30x20=600 GPH. The best thing to do is see how many GPH both the filters put out. If they dont reach these numbers then a powerhead would definitly help. Water movement is crucial to not creating any "dead spots".
As far as the cycling, the water will clear up, but if its fairly new and you recently added the sand, or the sand is stirred up, shut your filters off. The sand if gets caught in there can ruin your filters and pumps. Turn them on once everything has settled. Yes, the cocktail shrimp does work. Just get 1 dead, fresh, uncooked cocktail shrimp from the store and throw it in there. Some people take it out when the ammonia spikes, but others just leave it in through the whole cycle, its up to you. Please don't use fish to cycle. The poor water quality will probably kill them, and definitly burn their gills. And IMO I would raise the SG up a little, to around 1.024 to 1.026. Sorry for the long post
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