both my nanno and my duna are on a cold window sill. 62' yesterday. i keep a couple of ounces in the fridge to reanimate when i feel i might need it...the way bill described, i use bottled distilled and nuked old tank water. once in a while i setup a freshwater nanno culture on the side. this is handy for ato and auto feed. duna will convert as well. setting up subcultures is good idea.
i think working with other species is also good idea. tetra should be easy, myself, i couldn't get iso to take... i got a coupla gallons, but it crashed and didn't come back. you've inspired me to try so new stuff... when you see the different colours of green, it's kinda cool...
as for the clumping, i would physically stir your nanno daily until it comes back. try adding a drop per mil of ferts directly into a subculture, without the extra water.
here's some dunaliella cultures. the one in my hand is about 64', the one in the back ground kept about 76' ish. under par 30 and 20wfl
i haven't cleaned the bottle in possibly a year. it is duna under the microscope.
my nanno is at 64' right now as well.
i really don't think it's temp. it could just be that you need to get a good subculture going.