We've had the high phosphate levels for so long now, I can't remember what the last reading was. Interesting what you say about fertilizers though, as we put lots of tablets under the gravel before the tank was stocked with plants. The plants have always done incredibly far too well. I wonder if that solves the I going mystery of the last couple of years? Can't remember which make of fertilizer tablets we used now. I don't suppose there is an easy solution to the problem now either!
Plants should be using up phosphate, not increasing it. Other possible causes are rotting food or plant matter, high tapwater phosphate and some additives and fertilisers. High phosphates can definitely cause a problem for shrimp. What level is yours?
It won't have been the assassins that got the shrimp- those shrimp can move bloody fast when they want to! My cherry barbs are constantly trying to eat the baby shrimplets with very little success, no way could the assassins get them. What are you water readings? Shrimp can be picky.
Bamboo shrimp are big and impressive but they need quite a steady flow of water because of the way they feed. You might need to add an extra powerhead if your filter doesn't provide enough flow.
TBH I'd give something cheaper another go first before spending money on a powerhead and a bamboo shrimp. You can keep different kinds of shrimp together so you can always get the bamboo shrimp later on if you want.
Tried cherry shrimps a few months ago, but all got eaten (I suspect by our evil assasins!). Saw bamboo shrimps in the shop at the weekend & wondering if they would stand a better chance in our tank. 180 litre Juwel with far too many guppies & platies, 2 tiger snails & a few dozen assasin snails. Heavily planted. Views anyone?