strong fishy smell in tank

i have a 4x2x2 with a eheim 2260 filter and several sponge filters so very well filtered
i have a issue with a very strong fishy smell coming from my tank i have large bag of carbon in the filter also only replaced recently but still have a strong fishy smell
i also noticed when doing a water change a week or so ago i left my anubius plants out to long and they look very sick back in the tank should i just cut them back all the sick leaves
ph 7.5
nitrate 5
water temp 27.5 degress c

Leave the anubias plants and see how they go unless there are very obviously dead bits. Its unlikely to have any impact on water quality in such a massive tank assuming there aren't masses of them and anubias can be prone to rot when cut. The fishy smell is quite worrying though, and could be linked to deteriorating water quality. Whats your maintenance routine? Any ammonia/nitrite spikes? Missing fish or snails? If it was my tank I would test for everything, note any changes and even if that didn't flag problems I would still do a 50% water change to be on the safe side. .

It could be ammonia in the tank that is causing it.
Is your fish population still the same as before ???
Sometimes one can die behind a rock and go unnoticed.
But definitely a water change is in order. Just make certain the water is correctly dechlorinated and that it is at the right temperature when adding it.
Lastly I have noticed that blue green algae (seems like long tough strands of algae but are in fact Cyanobacteria) can have a strong smell that can be perceived as a fishy smell. Remove any that you find and srub rocks that have it. Cut back on feeding.
Let us know how you go.
Marty