Fish emulsion is ground fish.

sorry, i mean as far as nutrients?

I think that there could be some nutrients in fish tank water, but you would have to dump the fish also to get any fish emulsion from that.

Seriously, if you have a couple small trees, a bit of fish emulsion might not hurt. Myself, my trees grow well with or without adding compost or other oganic matter though a modest amount is good.

Tank water cannot be a substitute for fish emulsion, to answer your question.

Depending on a number of things fish emulsion might be good for plants, or it may not do much of anything for them. Fish emulsions usually are about 5 percent Nitrogen, 2 percent Phosphorus, and 4 percent Potash, or in that range. If the soil you are growing your plants in is deficient in these things then it probably will help, however if that soil is a good, healthy soil and supplies the soil bacteria that feed those plants with what htey need spraying fish emulsion might well be a large waste of your money.
Take a good, in depth, look at the soil your plants are growing in to determine what is needed before spending money on stuff you might not need which is neither good economic or enviromental practice.

Tank water will have very low nutrients compared to fish emulsion.
I'd use it on the garden, but not as a substitute for emulsion.

The most important difference is that fish emulsion is far more than tank water, it is actually pureed raw fish. It is, therefore, very high in nutrient levels compared to a pint of water mixed with piscine poop and pee. That being said, some cutting-edge gardening is being done with exactly what you describe, and if you Google aquaponics, Growing Power, or Will Allen, you will see just how effective a proper fish-tank water recycling system can be. In the meantime, it's probably best if you keep your pets away from the blender... your trees will probably prefer the bottled stuff, even if your nose knows better.