You know... I have had a 500ML bottle of Seachem flourish for like 3 months now for my 55G, 30g and 27g... and Its only half empty.. lol You only need to use the stuff once a week and its extremely concentrated..
The only liquid fert that does run out often is API leafzone.. I am on my second bottle now first was a 250ml and the second is a 500ml that's about 1/4 full. But that stuff again is only once a week.. The stuff that does get expensive is Liquid CO2... I don't even use the stuff anymore because of that.
How long have you had the sword for ? If its less than a month old... that's why its doing that..