25-30 fish does seem rather a lot but I think it depends on how big they're going to get. My gold fish are still fairly small after 3-4 years but I have one very big fish who I fear will outgrow my small pond in the next couple of years - he/she is still okay for the moment.
The garden centre near me also deal with aquatics, they're very helpful with advice. I also got a pond man (recommended by the garden centre) to come and look at my set up when several of my fish were dying but, as I said, I was cleaning the filter too frequently and my fish have been fine since I've been less zealous with the cleaning.
Not all my fish float when they die. A couple have died and stayed at the bottom of the pond. I believe it depends on the cause of their death, e.g. whether it's due to water pollution, lack of oxygen or disease. I haven't had any die for a few years now so I must have got something right - probably the fact that I actually do less now, feed the fish less, don't feed them in cold weather and interfere as little as possible with the filtration system (unless it obviously needs cleaning due to slowing down).