Platy At Water Surface

The reason is your nitrite level (and possibly ammonia though as you say you can't know if that reading is accurate because of the Prime). Those are classical nitrite poisoning symptoms; the nitrite level must be kept below 0.25 at all times to keep the fish safe. Wth 0.8, a 50% water change would only get it down to 0.4 which is still too high. You need to do an 80 to 90% water change to get your nitrite down. Then as many water changes, as often and as big as necessary, to stop the reading ever going over 0.25. I know that Prime says it detoxifies nitrite but their literature doesn't explain how. Unless they can give the chemistry involved (and I have a degree in chemistry which is why I want to know) then I would take the claim with a pinch of salt and do the water changes.


Speaking of salt, there is something you could try. Salt is the old remedy for combating nitrite poisoning and your fish can cope with it, so long as you still have only guppies and platies. The dosage rate is 0.1g per litre. By salt I mean pure sodium chloride; table & cooking salt usually has additives to keep it free running. Would you have access to pure NaCl at work? Otherwise, the rather expensive for what it is aquarium salt from the fish shop. This is not a substitute for water changes, they still need to be done to lower the nitrite level. And once the filter is cycled, you will need to remove it by several water changes before getting more fish as there are many species which cannot tolerate salt at all.

Which brand of test kit are you using? The one I use doesn't have a colour on the chart for 0.3ppm, so if yours goes 0, 0.3, etc, it may be what my tester would call 0.25.
That is the highest your nitrite should ever be allowed to get. I would still be inclinied to do a water change though as it is likely to go higher over the next few days.