What do you mean by dim? Light intensity? or length of photoperiod?
If it was me, I would shorten photoperiod to 6 hours, but give the plants full light intensity, ie. 100% brightness.
Dim lights benefit the algae.
H2O2 at 1ml per gal is ok.
I wouldn't use fertilisers, except for maybe root tabs, and even then only a very small piece deep into the substrate. Any other ferts will benefit the algae as much as the plants unless you take out as much algae as possible.
Floating plants can't hurt.
And as Simon said above, give it some time since this is a new tank. The plants in there have not rooted in yet. Once that happens, they should out compete the algae.
Got a picture of the tank?
Yeah, we just go by age of the shrimp. There is no visual indication until you see a saddle. Up till then it's just a guess.
Crystal shrimps are particularly difficult to gauge, since you can't see under the carapace until Cherry shrimps.
Parasites don't kill their hosts so quickly, it doesn't benefit the parasite. So you can rule that one out.
Diseases also normally take a while, certainly not overnight.
Quick deaths are either toxins/poisons or aggressive breeding apistos.
Either way, sorry to hear the news. Hope the Apistos make up for it with lots of babies.