What problems should I expect with hydroponic lettuce?

The biggest hydroponic lettuce problems are environmental and system based rather than classic field pests. Heat is the main enemy: warm water and air cause bolting, bitterness, tip burn and root disease like pythium, so cooling and clean water are critical. Watch nutrient solution strength and pH closely, since drift causes deficiencies fast, and guard against pump or power failure that stops flow. You can still get aphids, thrips and caterpillars in the structure, plus fungal leaf issues in stale humid air, managed with netting, hygiene and soft controls since it is eaten raw. Confirm nutrient management and any control products with an experienced grower and your KVK.