Trying to sweeten Liquid Fish Fertiliser?

Any idea how I can sweeten 200 litre drum of liquid fish fertiliser I've got going? Is in plastic cap sealed drum but has an odour.

Vent it through a charcoal filter. You can buy them at aquarium suppliers.

Thank you for that, was thinking more of what I can add to the liquid.

I aerate my Horse manure liquid fertilizer, with an aquarium aerator. Air kills the bacteria that causes the bad smell. I'm not sure how well it will work for fish though.If you try this be sure to use an rcd.
You could also try adding comfrey leaves as a sweetener but I think the rotten fish smell would be to overpowering. Still, comfrey is good to add anyway, if you have some.

A spoon full of sugar helps the fertilizer go down.

What do you mean by "sweeten"? Are you trying to improve the smell, or to change the PH so it's less acidic?
There is no way to improve the smell. Just dilute as per instructions, and water on. Try not to get it on any clothing *LOL*
If you're trying to increase the PH so it's less acidic (sour), wood ashes added to the mix will help.

EDIT: I keep my home made fertiliser out doors, next to the glasshouse. It smells to much to have in any enclosed structure, or too close to the house.

Some of the worst smelling liquid manure I ever made was using comfrey leaves.

Aerating it sounds like an excellent idea.
I had some dry high protein/nitrogen horse pellets that had decayed and were really stinky like urea. What I did with them was put them into my compost bins that were already half full, mixed and watered them in, then added a good layer of lawn clippings over the top to seal the pong from getting out and annoying the neighbours. It's kept the smell in, however I possibly created a low oxygen environment in my compost that is not good for efficient fast composting and in fact may have let the nitrogen escape back into the air in the form of methane.
Obviously whatever you do you need to retain as much nitrogen in your fish fertiliser compost as possible.

Isn't it lime that takes the smell away?

yeah but lime will also remove the nitrogen.

I don't remember any of my garden teas smelling sweet as such, but I never put fish or fish guts - or meat of any kind in it, relying on comfrey, horse poo, chook poo (limited), green kitchen waste, crushed eggshells, and seaweed when I could get it. It certainly boosted growth, I swear by the stuff, but it was always whiffy, and needed regular stirring, and storing well away from anyone who could be offended. I suspect fish and sweet might be a challenge too hard.

I've made liquid comfrey fertiliser and general manure fertiliser with a few fish heads thrown in.
I swear, the fish head fertiliser smelled worse. A really obnoxious smell that's impossible to ignore.

I did some Googling out of interest on this matter, all I could find is that it's better to bury fish solids in the ground then put lots of dirt over it so that hungry animals don't dig it up, and it makes excellent fertiliser and it doesn't smell.
I couldn't find any ideas on Google how to reduce the smell of fish liquid fertiliser but at the moment the smell is mostly contained right? So what if you use it in small quantities, apply it straight into the soil, or even in shallow trenches and then cover it over? This should also help to prevent flies. I would apply it on a day when the neighbours are working, and I would pay attention to the prevailing wind condition so some busy body elderly person up the street with nothing better to do than complain doesn't notice it.
Also consider applying it during or immediately before rain to reduce the observable smell?
Something I've done to reduce the smell of my chicken manure in their nesting house is to sprinkle Bokashi Zing mix onto the floor, it works; and Bokashi Zing mix can also be flushed down toilets into septic tanks to promote efficient digestion of the anaerobic sludge, so if I had a real problem with smell in this 200 litre of stinky rotten fish liquid I would take a chance with this Bokashi method. You need to borrow a cup of the Bokashi Zing from a friend otherwise buy a 1kg bag from somewhere like this http://www.zingbokashi.co.nz/product/compost-zing-single-bag-2/
Then you put it into your drum and fill it to the top with water to exclude all oxygen. The Bokashi anaerobic fermentation process produces mainly lactic acid so the smell is similar to vinegar (acetic acid) and not offensive. The lactic acid tends to deaden or transform the other rotten smells that may be present. A cheapskate way of doing this would be to throw some live yoghurt into the barrel and seal it up to remove all oxygen for a couple of weeks but Bokashi is formulated using several micro-organisms and definitely works better.