Unfortunately I cannot do co2, no cylinder and it being a 100g tank would be hard to overdose. I think I'm going to dose a ton of algae fix and hopefully it'll get rid of what I can't catch.
Bad idea, most of the algae medicines are just copper, and that will stay in the gravel and silicone and filter media, etc. And you won't be able to get much of anything in the way of shrimp to live in it for quite some time.
If anything I would just empty it out until the top of the substrate is dry and keep it empty for a week, to make it double kill put a co2 bomb in there (chunk of dry ice or vinegar/baking soda bowl).
I like the ammonia poisoning idea. One litre bottles are one dollar at my local dollar tree. I'd just turn IDF the filter and lights for the night and dump in some ammonia. Turn it all back on in the morning and let the plants and filter clean up the ammonia. Repeat two or the times to make sure it worked. Seems simple enough to me.