HBSterbai

Thanks. I poured it over the main couple of patches. I don't have a syringe on hand though. The tank contains dwarf neon rainbowfish, neon tetras, corydoras, and bristlenose plecos. Also I just noticed the bristlenose had bred, so a whole bunch of the fish are teeny tiny plecos.

Thanks for all the info.
NO2- (nitrite) - if you have any nitrite then it will be too much and will be 'burning' the 'skin' on the fish and damaging the gills eventually causing death.  Your NO3- (nitrate) is also a little high.
I suspect that the feeding is too much - once a day is sufficient or even once a day six days a week.  You could cut down on the pellets too as the waste will sink into the substrate.  The size of a fish stomach is approx. the same size as their eye.  Too much food, too much waste and your filter may be struggling to cope with it.  If ammonia or nitrite are present the fish will be coming up nearer the surface of the tank looking for more 'air' and the surface movement (gas exchange) will be providing better conditions for them.  Cutting down on the feeding will also be better for your wallet
Rainbows are very active fish normally, they like plenty of swimming space so having them 'hang' is not ideal.  They also have very tiny throats and require micro pellet sized food.  
Where to from here - I would hold back on adding any more medications to the tank and work on the water quality.  First up - to reduce the nitrite to 0 by a decent sized water change - 30 - 50% and then test again.  If still showing nitrite continue with daily changes of around 20% and if you do have any prime available add that to neutralise the nitrite.  Prime, while neutralising the effect of nitrite will still show a reading for it, so continue until the reading shows 0.  These water changes should also assist with the nitrate readings reducing.  Once your tank has shown 0 then reduce the water changes to 10% every couple of days for around 6 days and then stop and return to your usual maintenance.
Hopefully this will stop the fish deaths.  

I may be a little late commenting.  Furan2 also carries a warning hazard in that it is known to cause cancer in mice.  Please wear gloves when handling and putting your hands/arm in the tank.  

Furan 2 contains 2 forms of antibiotics - forgotten what ones but they won't effect the plants.  If I remember correctly the verdict was out whether the antibiotics in Furan 2 will effect the bacteria in the filter but if you use it to treat the tank just up the water changes for a couple of weeks after treatment.
The amount of water changed each time depends on your particular tank (size, stocking, amount feed, messy fish. effectiveness of filter etc).  Little and often is better for the fish then one big one now and again.  Best way to tell is to keep testing your Nitrate levels - create a chart of the measurements and if the level is increasing before each change then more water changes are needed.  The chart will tell you when you have it right when the Nitrate levels plateau at a low & safe level. 
 

I have a little less than 20 of these now. I'm really happy with them. I have never kept any rainbowfish before so its really interesting to observe their behavior.