Help with my fish!!!?

you need to check more then just the ph u need to test ur nitrate nitrite how hard ur water is and the and ur alkalinity tetras are very sensitive and need PERFECT living conditions it can't be off at all the reason ur gourami and pleco aren't effected is they are hardy fish they can take the changes in the water tetras are small and will die fast if the water is different then what they had been living in previously i suggest if thoose were ur first fish until u get ur water levels perfect try some swordtails molly or platies or some more gouramis if u perfer as they are all hardy fish and will help u in correcting ur water issues if there are any other thing is tetras need heated water so u need to look up the types u have and see what the temp is suppose to be at cause a few degrees to cold or to hot and bam they are dead and it happens quick 2 hope this helped sorry if u lose more fish if ur looking to get ur water right i use this stuff called prime fresh and salt water conditioner stuff smells like death but it works amazingly at getting ur water levels right i have a 10 gallon tank and in a week of putting in a ml of it once every other day the water levels are PERFECT!