A few things my friend.
Firstly, you can inject CO2 safely with a KH as low as 2 degrees. I know this as I, and many others (Amano for one) have done so. I stick to KH 3 to 3.5 now though but that's another story (pH related). In short your KH of 5 is perfectly safe.
Secondly, injecting CO2 will have virtually NO influence on your KH. CO2 only changes pH, this is how and why we can work out our CO2 levels from using pH and KH levels. Higher CO2 = lower pH. Lower CO2 = higher pH. Simple. I use this CO2 table.
http
/www.floridadriftwood.com/page.asp?id=16
Thirdly, you do need more CO2. I set-up a DIY CO2 rig at the weekend for a friend with a 70 Gallon and he is getting 30ppm!!
The whole lot cost less than £5 ($10).
You want to know how right?
You need -
2 litre plastic fizzy drink bottle with cap
Silicon sealant
6mm drill or similar
3 metre silicon airline/tubing
lots of sugar
dried yeast
bi-carbonate of soda
teaspoon
funnel
weighing scales
measuring jug
water
1. Drill 6mm or slightly less in bottle cap.
2. Insert airline an inch or so in hole and seal with sealant.
3. Allow to dry.
4. Recipe - 400 grams sugar, 2 level teaspoons yeast, 4 level teaspoon bi-carb, 1.6 litre tepid water.
Pour in sugar, add yeast, add bi-carb, add tepid water. Use funnel if necessary (less mess).
5. Attach airline to your Nutrafin diffuser.
6. Watch CO2 bubble away after a few minutes.
This mixture should give a fairly constant output for approx 7 to 10 days.
You could obviously make 2 or bottles up and couple the airlines to the same diffuser using a 2-way splitter.
Good luck.