I bought fish online but I noticed it had tripolyphosphate additive.
Is there some way I can at least reduce the content without compromising the fish?
I was thinking soaking the fish in water as it thawed or boiling it for a short time just before baking it.
You can remove any salt like this by diffusion/osmosis - basically by soaking with multiple changes of water over a period of time. This technique is used to make preserved salted-fish edible.
How long you need to do this for will depend on the thickness of the fish and the concentration of the salt. You can determine this empirically by experimenting, but you need some way to measure how much salt is coming out at any one time (e.g. in a change of water), which will be difficult to do without some analytical chemistry skills.
However, in your case, you have a fish that has had some small amount of salt added to improve texture for keeping it longer. The amount of salt added is not enough to preserve the fish and prevent it from rotting, so soaking it comes with the added risk of causing the fish to spoil. If you are going to do this, keep the soaking fish in the fridge. This will slow the diffusion process somewhat. It is also likely that it will alter the texture and probably the flavour of the fish and result in soggy, not so great tasting fish.
