I have previously used Excel in tanks with Vals and had no ill-effects. I wouldn't say 100% that it will negatively effect them.Excel kills some plants like valisneria & anacharis. It's not a fertilizer as such, it's used as an algaecide & plants can use it as a sort of co2 substitute. Be careful not to OD your tank, it's also a medical disinfectant in slightly higher concentration than Excel is sold.
You really don't need the lights to be on as long as they are; however, if everyone is happy in the tank and you don't have crazy amounts of algae don't change it. Having a planted tank is all about finding the right "balance (ferts/flora/fauna/lights/photoperiod/etc)" for YOUR tank. The "balance" can change as fauna increases/decreases/feeding amounts change, flora grows/gets trimmed/changed, etc... but once you have your tank balanced out don't monkey around with it just because. I have found that the more I screw around with a tank the more unhappy I am with the results.Ok, what sorts of problems? I've never had much of a deal with algae in the tank.
Thanks,
Brian