Hydration. Fiber is important, but it alone will clog up the pipes. Fiber is hydrophilic, it absorbs water, so you need a certain amount of water to go with it.
If you're hitting your hydration goals, you just need to add 200-300mL of water per meal.
If not, hit your WHO hydration goals of 2.2-3 liters of water per day depending on weight.