problems with new tank

Question
we set up a 226lt aquarium on 18/12/06. since then we have added 4 zebra danios 7 male guppies and 3 suckers (i think they are bristlenose catfish). The ammonia and nitrite has stayed at zero from day 1 and the nitrate has got to 5ppm in the last week. The pH seems to stay around 8.2 which seems high, the tap water is 7.6. I can get the pH to drop to 7.8 if I do a water change of 30% every day but it shoots back up within 3 days if I stop. The tank has a fluval 4plus filter and a 300 watt heater along with an air pump running 3 airstones and an ornament. I do a 25% water change every 5-7 days and add aqua plus and cycle. Its a silver sand substrate (which i try to clean whilst doing a water change)with lava rocks and 3 types of plants. Is the pH a problem, as we are hoping to add some neon tetras next month. Also 1 of the danios died of unknown causes last week (got water samples checked all ok)and since then several of the guppies have had damage to their tails and hide among the plants lying on the tank bottom. Could this be the danios bullying them as they do chase each other and sometimes they chase the guppies. 1 guppy has lost nearly half his tail but doesn't seem scared as he is always swimming around but 2 of the long tailed guppies are always hiding but do come out to feed. I have noticed they have had red ends on their tails which goes within a day. I clean the filter, when i do a water change, with aquarium water, they are fed twice a day with flake food which is all eaten within 3 mins. Thanks in advance.

Answer
Hi Martin,
It sounds like you are doing everything right. I really don't know if your zebra danios are being fin nippers or not. It is possible. Here is a website that explains that-http://www.fishlore.com/Profiles-ZebraDanio.htm It is possible that your male guppies could be fighting and biting each others tails.  I've had mine do this at times.  I don't think the ph will bother them since they are usually pretty good about adjusting to various ph levels without problems.  Just observe who is doing the biting. It could be that they all are contributing to the missing tails.  You are doing such a good job with your tank I'm sure you will be able to pin point who is responsible for the fin damage.
Hope this helps,
Karen