Is trading in fish feed permissible, given that one receives a check guaranteeing the principal and large profits every 3 months, bearing in mind the concern that the trade might be illegal due to the enormous profit?

Paying money to someone to trade with it is called Mudarabah (definition) (profit-sharing partnership), and it is permissible under certain conditions. These include: specifying a known share of the profit, such as half or a third; and that the loss is borne by the capital provider, unless the agent transgresses or is negligent—in which case the agent only loses his effort. The agent does not guarantee the capital. If the capital provider stipulates a guarantee for the capital, this condition is legally void. A high profit alone does not indicate the impermissibility of the trade. As for trusting the person who trades with the money and taking the necessary precautions to safeguard rights, this requires consulting experts and trustworthy individuals.