You really go 8 hours with out feeding them? Also make sure you Ph is really at 5.6, because ph pens need to be calibrated quite often, and if ur any lower then ur going to get lock outs, happened to me, and u get dark rusty spotting like i see here. plants like low ph for calicium, and high ph for magnesium, u fall to low and ur certain to get an MG lock out from the calcium. If u havnt changed nute concentrations then its not an under or over feed. If do like ur supposed to and add a little more each week, then no way ur going to get burns like that, that quickly. Routine res changes? unused nutes build up.