The use of ethoxyquin (EQ) as an additive of feed intended for aquaculture will be illegal from 1 April 2020. Such feed may be continue to be used for another three months after this date i.e. until 30 June 2020. Anyone farming fish outside the EU should be aware that feed containing EQ continues to be legal in many parts of the world, and they should take precautions to avoid the use of this feed in product intended for the EU market.

Looking at RASFF, there have been 6 notifications for EQ since 2005. Only one of the notifications concerned fish (in 2005). This suggests that there is no push at the moment to test seafood for EQ. I am not aware of any plans by the European Commission to increase its testing for EQ in domestically farmed or imported seafood, in relation to the ban on EQ. If the EU or even individual MS see EQ as a threat to health, there may be increased testing.
 
There is no maximum acceptable level set for EQ (or its metabolites and associated impurities). There might at worst be zero tolerance. Authorities in the EU might use EFSA’s Acute Reference  Dose (ARfD) for EQ of 0.5mg/kg bw/day to arrive at a tolerable maximum residue level. However there are problems with this: the main toxicological worry comes not from EQ itself but from EQ’s breakdown products (in particular ethoxyquin quinone imine) and from the manufacturing impurity p-phenetidine. Both are suspected mutagens.

Commission Implementing Regulation (EU) 2017/962 of 7 June 2017 suspending the authorisation of ethoxyquin as a feed additive for all animal species and categories is available here: https://eur-lex.europa.eu/legal-content/EN/TXT/?qid=1579852581573&uri=CELEX:32017R0962