Reservoir change?

[Stan]

At the moment I'm using a cooler which holds 15-17 gallons of water. I'm growing 10 tomato plants and 1 watermelon plant using a PVC fence rail with a 1/2 PVC pipe with micro sprayers. For the first 2 weeks I completely flushed and changed the water/nutrients 5-10-25 & 15-0-0. The plants are all getting bigger and I've been adding 3-4 gallons of water and nutrients daily. Should I still flush out and do a complete change of water and nutrients weekly, or can I do without it for now?