from Hk airport no problem...you are departing lar...tara check one. you turun which airport? klia ah, then no worries lar, as i say visit malaysia year, tara check , but if kana check , then rm50 faster faster come out lar...but usually wont check , if they ask you whats in your bag , just tell them fish, better be honest and usually they will let you go.
tips...for water plants better to put them in plastic water bottle, no need water , just as long they are wet, and dun open till you get home. actually HK , i dun see any good plants , mostly green var, nothing special actually and they are not cheap hor, especially if you buy from goldfish street or tung choi kai in mong kok. u want cheap and special var plants , then go to guangzhou....about an hour train ride from hk.

btw ..if you are planning to go overseas just for the sake of buying fish and water plant...then Jakarta and Bangkok is a better choice.
in jakarta , go to jalan sumanep, just behind jalan jeneral sudirman, sayur arrive on wednesday evenings, so go on fridays. good buys are java ferns and riccia, cheap like shit, 15,000rupiah per 12x12inches mat,
in bangkok, its in chatuchak, sayur arrive early sunday morning, plenty of red species plant usually 30baht will get you a 5 bundle pack. the more exotic ones are abit expensive but worth it since kenot find in malaysia. but spend time looking for fertilizers, thai ferts have fantastic results

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