Effects that modify a creature’s power and/or toughness will apply to the creature no matter when they started to take effect. The same is true for any counters that change its power and/or toughness.

Ichthyomorphosis overwrites all previous effects that set the creature’s base power and toughness to specific values. Any power- or toughness-setting effects that start to apply after Ichthyomorphosis becomes attached to a creature will overwrite this effect.

If the affected creature gains an ability after Ichthyomorphosis becomes attached to it, it will keep that ability.

Because damage remains marked on a creature until the damage is removed as the turn ends, nonlethal damage dealt to a creature may become lethal if Ichthyomorphosis becomes attached to it during that turn.

Ichthyomorphosis overwrites all colors and creature types the enchanted creature has. It’s just a blue Fish. The creature keeps any supertypes (such as legendary) it has, as well as any other card types it has (such as enchantment).

Ichthyomorphosis may enchant a permanent that is only temporarily a creature, such as a Vehicle. If this happens, Ichthyomorphosis will be put into its owner’s graveyard as a state-based action once the creature stops being a creature.