Maxigain has changed their tnc wef Dec 2019. No longer attractive. If you bother to read the posts here for the last 3 mths you would see most people have closed the account and move funds elsewhere .
If have 70k to 150k interest is about 1.4% or so and will fluctuate as part of it is tied to SIBOR. Plus cannot withdraw more than the interest monthly or else the counter will drop and hence interest will drop.
https://blog.moneysmart.sg/savings-accounts/citibank-maxigain/
Now to answer your question, 150k is the max not the min. Best interest (about 1.4%) for this account is 70k to 150k.
No need to pump in more every month. Just initiate deposit will do.